This is a small code I did some time ago for my project and I thought to upload it as a mod.
With this mod enabled, when an unit is upgraded it loses all the bonuses earned previously:
XP is reset to "0"
Level is reset to 1
All the Promotions are deleted (this can be modified in 'Promotions.sql')
The fact is that I didn't like having incredibly powerful tanks because my knights fought a lot of battles in the middle age. It simply doesn't make any sense!
Anyway, although the idea may seem radical, it eliminate a cumulative bonus that only the human player fully exploit, making the game fairer, in my opinion.
If you notice any bug or you have comments/questions, write in the thread.
v.2: halved the XP required for level advancement. (EXPERIENCE_PER_LEVEL = 5)
Upgrading Reset Units 2016-10-05
Upgrading Reset Units