Originally Posted by
Shoebopp
It may also be possible that the unit you want to retrain originated from the same "pool definition" from descr_mercenaries.txt. For example, if you're Ptolemaioi and hired a Mercenary Phalangite from Egypt's start-of-campaign mercenary pool (3 units, 0.12 replenishment rate), you can only retrain that unit in Egypt as long as that 3x0.12 pool definition is active. Once the Thureos Reforms takes place and that pool definition is replaced by 2 units, 0.08 replenishment rate, you won't be able to retrain that Mercenary Phalangite unit originating from the 3 unit, 0.12 replenishment rate mercenary pool. I got to this conclusion when, during the mid-game, trying and failing to retrain a mercenary unit I hired at the start of the campaign.