The impact of retraining on experience depends on whether you

EITHER retrain several units at once in one turn (or are training a new unit in the city) - experience decreases as green recruits are mixed with the veteran troops. Example: retraining three two-bronze-chevroned units of hastati reduced to half of their original strength will probably give you three full-strength one-bronze-chevroned units of hastati i the next turn.

OR retrain a single unit and are NOT (re)training any other unit in the city in that turn - experience remains as it was. It is probably so to represent that the veterans are thus given enough time to train the greenhorns and pass their experience to them. Example: retraining one two-silver-chevroned unit of slingers reduced to mere five men will give you one full-strength two-silver-chevroned unit of slingers next turn. It might be considered as exploit under certain circumstances and reasonable and justified use of this feature is advised.