IIRC the only unit that gets 0 exp replacements, is the type that auto-retrain (General's Bodyguard) over time. Thats probably due to the fact if you have 1 bodyguard with 3 gold chevrons, its not very fair for you to regenerate more 3 gold chevron guards for free every turn (not to mention that unit would be more of an assault unit then a protection unit).