The evidence is there. There is no costincrease for an experienced unit, yet it gets the recruits at the level of the unit average. This should of course be on the list as it makes the game too easy for us (we can easily get those high XP units while the AI tends to let them get killed off in longer campaigns).
Kraxis, I agree that there should be a higher cost for retraining high XP units. However, I've noted AI take greater advantage of the feature that the new recruits are at the level of the unit average XP, specially romans who manage to get their units depleted to low 20s or 30s in autocalc with other AI factions while gaining lots of XP for the remaining soldiers in the said unit. The AI then manages to retrain many of these depleted units (mostly by then they are down to 10 or so left in the unit with 3-6 XP on average) and ends up with stack of high XP units towards the end of the game. This is however, true where the player does not engage the Romans or the concerned major power till late in the game. Like in my case, I played Parthia and didn't engage the Romans for the first half of the game. When i did, they had so many high XP units (and not because of the temple upgrades only, because Julii don't get such XP upgrades from their temples). Anyhow, I am somewhat in favour of keeping this current system, though with an increased cost for retraining.