Without Infinite XP, you are only allowed to keep 25 exp after any upgrade. Note that upgrade is different from promoting or leveling up, THAT shouldn't be adjusted on units. But when you upgrade and the unit takes on a new form, if that unit had more than 25 xp, it now only has 25 and you've gotta re-build yourself back up across the great divide all the way to the next level which could mean that unit is now never going to promote again. I usually get my cities building units out of the gate with more than 25 xp in C2C so its particularly frustrating if I build a unit and then have to upgrade it soon after so it takes much more planning without Infinite XP.
Additionally, those promos that are required to allow your units to bypass a certain level of experience gain from barbarians and animals are necessary to bypass 20 xp from either source. This means that once you're at 20 xp, if you don't have the Anti-Barbarian XP unlimiter promo, you will not gain any xp from defeating a Barbarian, and if you don't have the Anti-Animal XP Unlimiting Promo, you won't gain any further xp from defeating animals. Make sure you promote to these on any unit you want to get any benefit from defeating barbs and animals (omg this option off is SO annoying!!!)
(Oh, crud... @Koshling - THIS is why I had worked out re-training the way I had initially, without dropping the level and allowing the xp on the unit to indicate that the unit should now be leveling... without Infinite XP it means you won't HAVE the xp to retrain in a lot of cases... oh well, I guess without Infinite XP if you lose a promo you should've been able to retrain you'll have to do it the hard way and re-earn the xp now... Just another element that should be enjoyed by those who prefer having weaker overall troops in the game.)
Why would you even reply like you did if you didn't put your own slant to it? Do " " marks = cynicism? I don't think so. 
