A retrained unit doesn't retain the experience. Instead the experience reduces in the amount of new soldiers. I have seen units going down from golden chevrons to bronze ones by retraining.
I also don't think it is absolutely unhistorical for a unit beeing refilled with new soldiers (not in zero time, but in fact it takes one turn exactly as recruiting a new unit if you don't retrain many at once). It is one thing in an army like the early roman ones that is gathered, goes to war and is disbanded afterwards, but another in a professional army like the late republican and imperial roman army. The legions were not disbanded alltogether anymore, but were permanent units in which the soldiers came and went. These units were indeed filled with new recruits when the numbers have been reduced through casualties or retiring.