Since I apparently run out of space in the mod description, here you will be able to see the full list of things with their explanations and tips, meanwhile mod description will contain only the basics.

Upgrades:

Dozer Blades for Leman Russ Tank variants (Demolisher, Executioner and Vanquisher), Manticore, as well as Rhino
(all part of Unit Pack by dexgattaca).

Dozer Blade to my simple Bane Wolf and Hellhound.

Pintle mounted storm bolter (gunner) to Rhino
(Requires Pintle-Mounted Storm Bolter upgrade which is part of HOL Astartes unit add-on)
(Rhino is also part of Unit Pack by dexgattaca).

Predators (Annihilator and Destructor) can now have their sponson heavy bolters upgraded to lascannons - specializing them even further into the anti-tank role but making them less efficient against infantry.

Track Guard upgrade/tech (Tier 8) - Grants Track Guards which increase armour to Centaurs, all Chimera variants, Salamander Scout, Trojan Support Vehicle, Hydra and Wyvern, Devil Dogs, Hellhounds and Bane Wolfs, Basilisks, Manticores and all Leman Russ Tank variants including the Tank Commander.
(Original mesh and textures are used by Leman Russ Demolisher which is part of Unit Pack by dexgattaca)

Catachan Devils and Jungle Fighters can now also be equipped with smoke grenades to create smoke screen -> Smoke Launcher (Tier 7).

All Tauros Rapid Assault Vehicle variants can be now upgraded with Hunter-Killer Missiles and Camouflage Netting
Both Tauros RAV and Venator variants can be now upgraded with Relic Plating.

Thunderbolts (standard variant only) can now exchange its Skystrike Missiles for Tactical Bombs turning them into a Fighter-Bomber unit type.

Basilisks can now shoot their Earthshakers using extra charges, which will deal extra damage and have higher both min and max range but also cause extra wear and tear on the barrel, dealing some damage to itself in the process.
Basilisks can now also use Smoke Shells, which will allow you to cover a tile in Smoke Screen from afar.
Use your Smoke Shells to either aid your frontline units in need from afar or to cause distruption among your oponents by covering their field of view with smoke.

Trojan Support Vehicle has now an ability to clear Fallout.
If you mess up some place with an Atlas Atomic Bomb now Trojan can help you out in clearing up your tiles faster from the Fallout.

Centaur Light Carriers can now 'Tow' your units with Field Gun classification trait (infantry artillery pieces), granting them extra movement and relentless trait [Can only be used if the target unit didn't move in this turn yet].

Unit Specializations:

Hellfire Dreadnought (part of Unit Pack by dexgattaca) can now be specialized with more weapons such as:
Twin-Linked Autocannons, Assault Cannon, Multi-Melta or Plasma Cannon in the same way as standard Dreadnought in the HOL Astartes unit add-on.

Dreadnought can now also replace its Multi-Melta for dual Heavy Bolters
(Unlocked already with Dreadnought upgrade/tech).
Dreadnought with Heavy Bolters has lower requisition maintenance making it great option to save your resources.

Stormraven Gunship can now replace its twin-linked assault cannon for twin-linked lascannon or twin-linked plasma cannon, all variants can also replace their heavy bolters for twin-linked multi-melta.

Stormtalon Gunship with Heavy Bolters -> Launch Pads (Tier 7)
It's slightly cheaper and faster to produce variant than standard Stormtalon with Skyhammer missile launcher.
It's more effective against larger groups of light infantry than other variants but worse against other kinds of enemies.
Either in a city or an outpost it can exchange/upgrade its heavy bolters into Lascannons or Skyhammer missile launcher.
Lascannon variant is most effective against vehicles but worse against other kinds of enemies than other variants.

Tempestus Scions can now be specialized/equipped with more weapons such as: Hot-Shot Volley Guns, Flamers, Melta Guns, Grenade Launchers and Plasma Guns, after researching More Weaponry for Tempestus Scions (Tier 6) upgrade/tech - requires Reinforcement Pack DLC.
Kasrkins may be equipped with Hot-Shot Volley Guns as well.

Death Korps Heavy Bolter squad can now also be specialized with Heavy Flamers -> More weaponry for the Death Korps (Tier 4) - Demolition Pack DLC required

Chimera can now also be outfitted with Heavy Flamer as its main weapon -> More Weaponry for the Chimera (Tier 3)

Devil Dog can now be specialized into Hellhound or Bane Wolf.
Both are great anti-infantry options which also do damage over time (Burning/Blight),
Hellhound's inferno cannon has 2 range and better armor penetration in direct damage meanwhile Bane Wolf's Chem Cannon excels against targets such as Enslavers or Chaos Spawns and has better damage over time.
(Tip: you can apply both blight and burning to the same unit, as such using Hellhounds and Bane Wolfs in tandems will highly increase the damage over time you can do).

Scout Sentinels can now be outfitted with more weapons, such as: Heavy Flamer, Autocannon, Missile Launcher, Lascannon or Plasma Cannon -> More Weaponry for Scout Sentinels (Tier 3)

Armoured Sentinel with Multi-Laser -> Armoured Sentinels (Tier 5)
It can also be outfitted with more weapons, such as: Heavy Flamer, Missile Launcher or Plasma Cannon -> More Weaponry for Armoured Sentinels (Tier 6)

2 New variants of Tauros Rapid Assault Vehicles, with Heavy Bolters and with Autocannons -> Tauros Rapid Assault Vehicle (Tier 3)
1 New variant of Tauros Venator, with Twin-Linked Multi-Lasers -> Tauros Venator (Tier 4)

Elysian Drop Troops can now also be equipped with Shotguns, Flamers and Grenade Launchers -> Substructure Extension (Tier 4)

Tempestus Aquilons can now be specialized/equipped with Melta Guns, Plasma Guns and Hot-shot Long-Las, after researching More Weaponry for Tempestus Scions (Tier 6) upgrade/tech - requires Reinforcement Pack DLC.


Unit Variants:

Predator Destructor -> Predaors (Tier 4)

Land Speeders Tornado -> Land Speeders Tornado (Tier 4)
Their standard version is equipped with heavy flamer, which then can be upgraded to assault cannon.

Tactical Scouts -> Scouts (Tier 1) - Escalation Pack DLC required
They are equipped with Boltguns by default but can also be equipped with Stalker Bolters and Flamers or Plasma Guns after researching Extra Scout Gear (Tier 6) upgrade/tech.

Devastators with Autocannons -> Devastators (Tier 3).

Land Raider Terminus Ultra -> Land Raider Terminus Ultra (Tier 9)
It sacrifices all of its troop carrying capacity for a superior anti-tank fire power, it also gets hot while firing (damages itself).

Sternguard Veteran Squad -> Sternguard Veterans (Tier 6).
Elite infantry unit that can be specialised with Combi-Flamers, Combi-Meltas or Combi-Plasma.

Standard Heavy Weapons Squad changed into Missile Support Squad
Heavy Weapons Squad (Heavy Bolters) and Anti-Tank Squad (Lascannons) -> Heavy Weapons Squads (Tier 2)
Fire Support Squad (Autocannons) and Anti-Armor Squad (Plasma Cannons) -> More Heavy Weapons Squads (Tier 4)
All heavy weapons squads fulfill the role of field fire support for the regular infantry to deal with various enemy types thrown at them, although they are better in the defensive rather than in the offensive.

Mortar Support Squad -> More Heavy Weapons Squads (Tier 4)
They serve as a long range, barrage fire support for the infantry, they are vulnerable in close range though, and just like other heavy weapon teams they are better in defense than offense.

Added Lightning Fighter and Avenger Strike Fighter to Astra Militarum - Adepta Sororitas DLC required
Lightning Fighter (Tier 7)
Avenger Strike Fighter (Tier 9)

Thunderbolt Fury -> Thunderbolt (Tier 8)

Marauder Destroyer -> Marauder Bomber (Tier 9)
This variant will obliterate ground targets with overwhelming direct firepower, decreasing its bomb load for 6 Hellstrike Missiles and outfitting its front turret with 3 sets of Twin-Linked Autocannons and rear turret with Twin-Linked Assault Cannons.

Marauder Colossus -> Marauder Bomber (Tier 9)
This variant is only lightly armed, completely removing its dorsal turret and bomb load for a single but extremely powerful Colossus Bomb, it will cripple or totally obliterate almost any single ground target.

Marauder Vigilant -> Marauder Bomber (Tier 9)
This variant is a surveillance, reconnaissance and comand aircraft, it will boost nearby allied Astra Militarum units and debuff any nearby enemy units when they are performing an overwatch attack.

Marauder Nuclear Bomber -> Marauder Bomber (Tier 9)
This variant is armed with only 3 sets of Twin-Linked Heavy Bolters on its turrets and exchanges its bomb load for a single Atlas Atomic Bomb, which will completely destroy and irradiate an area around the tile it will be dropped on (watch out for friendly fire!)

Atlas Recovery Vehicle -> Atlas Recovery Vehicle (Tier 4)
This unit can help you fix your lightly damaged vehicles (above 40% health) or Tow your badly damaged (below 50%) field guns, artillery pieces or lighter tanks (without the super-heavy trait)
[Towing increases movement of target unit by 3]
Use your Atlas vehicles to ''tow'' your damaged tanks/artillery back into safety or to grant them more thurst for their final charge.

Thunderer Siege Tank -> Thunderer Siege Tank (Tier 5)
This Tank is only armed with its Demolisher Cannon (and pintle-mounted heavy stubber if researched) but can be researched faster and is significantly cheaper than the Leman Russ Demolisher which role is Thunderer supposed to fill for. It also has an increased armour.

Earthshaker Platform -> Earthshaker Platform (Tier 7)
An earthshaker cannon fixed in a defensive point, can be constructed by tech-priests engineseers after researching it. Use these platforms to form a defensive line around your cities together with heavy turrets or place them behind the frontlines to barrage your enemies from fortified positions.

Earthshaker Carriage Battery -> Earthshaker Carriage Battery (Tier 4) - Demolition Pack DLC required
A man-portable earthshaker cannon, slow and heavy will require to be towed by a Centaur to be fully efficient, it's cheaper than Basilisks though and the crew not being limited by the confines of a vehicle can achieve better accuracy.
[As a Field Gun it counts as an infantry unit, you can use that to your benefit to make it even more accurate than Basilisk].

Note: A lot of smaller and specific changes, like upgrade/tech name or description change or buffs and nerfs can be seen in the mod changelog