Just another idea..1 player sends another 2 spearmen, other sends a praetorian in exchange ..
2 is not the right answer , but that is just a rough estimation. Exchanging troops could only be in equal value with larger amount of units being exchanged.
To not let 1 player "buy" himself into greater damage total, then only equal amount of damage spectrum could be exchanged while keeping in mind that tribes train troops with different speed and resource cost.
A limit on troops switching could ease the issue of not messing that one up, but lets say a teuton switches his TKs with TTs. Is he getting too great raiding boost ..or could gauls have to be able to trade their swordmen against clubs too, so everyone has equal chances? Seems complicated to find out the exchange rates, but the idea remains...
Could players exchange troops?