The manual states retraining reduce the units experience. This sounds logical but this is not true and I consider it a bug too. Because the retrained unit retains its experience and this creates a serious disbalance. I recently looked more carefully at my emergence service stack of hospitaliers and... all the units are golden! Without using the merging exploit (use the unit with the highest experience to merge in the others, but don't use it entirely - retrain it and use it for merging etc...) - so as I said without using the merge exploit, but just by winning battles and retraining. Now they are unstoppable and easily win against every stack in non-siege battles.