Update 3:
- Updated to clarify that undead and arethi troops are not listed due to not having upgrades. I didn't forget them, they just can't be upgraded into.

Update 2:
- Apparently, none of the Undead nor Arethi Elves' Temple Units can be upgraded into. On the other hand, the Svarts can upgrade their units into Temple Units, but those are from Warlock 2... soo... yeah...
- As a result of this new information, I'm overhauling the guide as I have time.

Update 1:
- As I've been playing my latest game, I'm updating the guide. Neither Paladins of Death nor Krolm's Housecarls can be upgraded into, so I've updated that information.
- Added a section on Version History.

In Warlock: Master of the Arcane, you often upgrade your weaker units into stronger ones. For example, Skeleton Archers can become Skeleton Snipers, Trolls can become Old Trolls, and Mages can become Magisters. This is done by both having the pre-requisite buildings built in a city you own, and then by clicking the gold star in the upper left of the unit's portrait that you which to upgrade and have selected. The cost is the same as buying a new unit, but you keep all your experience levels, purchased equipment upgrades, and enchantments.

Not so often noticed is that some units can be upgraded from your basic troops into ones normally purchased by building temples to a certain god and within a certain race's city. For example, Human Warriors become Human Veterans, but they can also become Paladins of Dauros. This means that if your starter troops you built way back at the start of a long game are now too weak as Advanced troops, you can keep all their extra experience levels, upgrades, etc. and upgrade them into Elite Temple troops.

Of note is that you must have upgraded a unit into it's first upgrade before it's second upgrade can be purchased. For example, Human Warriors can be upgraded into Human Veterans, and then into Paladins of Dauros or Blademasters, but Human Warriors cannot be upgraded direction into, say, Blademasters - you'd have to upgrade them into veterans first.

This section is the real reason I made this guide. As of making this, I cannot find a comprehensive list of which normal units upgrade into which temple units. So, I'm making one with what I know and have testing out, and I'm leaving it to the community to help me finish this faster by chiming in with whatever upgrades you know. So, without further ado, here it is:

Note: Undead and Arethi cannot upgrade lower grade units into Temple Units. As such, they are not listed.

Agrela (Life) -
Priestesses of Agrela come from Clerics or Healers (1)
Paladins of Life come from Ratman Robbers or Ratman Pirates (2)

Dauros (Order) -
Paladins of Dauros come from Warriors or Veterans

Helia (Sun) -
Archers of Helia come from Hunters or Rangers
Wolves of Helia come from Noble Werewolves or Court Werewolves (1)

Krolm (Strength) -
Blademasters come from Warriors or Veterans

Krypta (Death) -
Sisters of Krypta come from Clerics or Healers (1)

Fervus (Chaos) -
Beastmasters come from Hunters or Rangers (2)
Druids are still unknown

Lunord (Moon) -
Assassins come from Rogues or Cutthroats

Grumgog (Curses) -
Champions of Grumgog come from Goblin Archers or Goblin Sharpshooters (2)

(1) These upgrades may only be a sidegrade - for example, Healers are already Elite units, and thus recieve little or no benefit from becoming a Temple unit.
(2) These upgrades could use some verification.

If anyone has any corrections or more information, please leave a comment below, and I'll try to keep this updated. I am still working on finding all the upgrades, but since I almost always play max map size and number of foes, it's pretty slow to finish these up. Still, I'll get to it sooner or later lol.

I just wanted to include in this section any special tips I might come across that are related to Temple units, especially if it concerns their upgrades.

• If you aren't shooting for an Avatar victory, which requires a relationship of -100 to an apposing god, them consider manipulating the favor of various dieties in order to gain more than one temple unit type. For example, if you build a temple to Dauros, your second temple can still be potentially built to Krolm.
  
  
• Choosing which quests to purposely fail or succeed in can further aid in manipulating divine favor. It's a little tricky, but doing these subtle manipulations of divine favor can allow you multiple types of temple units. Just remember that you can't make use of those temples while significantly out of favor (-10 or worse I think), but already purchased temple units will remain under your control. Also, divine spells require divine favor, so that may play a factor as well.
  
  
• Some gods are closer in alignment to each other than others. Even if you're a devoted follower of Krypta, for example, you won't lose much favor by building a temple to Krolm relatively early on.
  
  
• Each god's spells have a certain strategy to them. Learning these strategies and using the ones that mesh well with your own strategies or personal playstyle can really help. For example, Lunord offers some mobility and can flood terrain, Agrela has a lot of healing and food/population buffs, and Dauros has some hefty anti-undead powers and the ability to create frozen lands (which is great for gold production), and so forth.
  
  
• Personally, I think Arethi Elves and Undead cannot upgrade their early units in order to balance out their already powerful units. Undead can, for example, use Elder Vampires to great effect against most troops fielded by enemy Great Mages. Arethi Elves, on the other hand, can make use of their early game units even in the end game. For example, their archers can earn a stun power by leveling up, which allows them to lock down most enemy units long enough for your other, more powerful units to get around to cleaning up enemy troops. If that's not cool enough, that same power (mind blast, I think) does spirit damage. Ouch.
  
  
• For reference, I'm including the alignment chart below. Any god that is next to another god that you're working for can be worked with without severe penalty. Working two steps out is also possible, but sacrifices the option to get higher end divine spells due to a lower overall rating. Beyond that, you just can't keep them all happy. Of course, you can still build a temple to one god, tick them off, then build a temple to another completely different god, but by the time you get your third or fourth temple built like this, you've probably already got the end in sight for even the longest games.