This mod gives Regiments of Renown the ability to gain XP, gain ranks, and gain stat bonuses.

You will see that even though they added the "rank 9" icon in battle, that this is purely VISUAL. When on the campaign map you can check their XP bar, and because they receive the XP AFTER the battle, they still gain the ranks and therefore the improvements to stats, and visible increase in chevrons/XP on the campaign map.

As for the "Rank 9 Icon" in-battle, as I mentioned, its basically not even real... I don't really know why they did it like that, it seems pretty odd to just add it visually and not make them "True ranks". I get that they were trying to reinforce the idea that they are veteran units, but the way it was handled, ranged RoR's don't even get their bonuses to Accuracy and Range like a TRUE rank 9 unit does.

FAQ:

Q: They have the Rank 9 Icon in battle, but gain XP on campaign, is this correct?

A: Yep, that's right, the Rank 9 Chevron is hard-coded to the unit in battle, but luckily for us, XP is calculated on campaign map, which is why campaign skills like the Thane are able to increase the rank of units in their lords army just by embedding, for example. So while they will always have the rank 9 icon in battle, they will be gaining ranks as you'd hope on campaign map!

Q: Does this mod work with your Regiments of Renown! unit mod?

A: I've created a separate mod for that, but it does exactly as you'd expect and lets the custom RoR's gain XP just like the vanilla ones!