What I did was either:
Start a new game as catholic, move_character near the pope, bribe the pope o perfect, declare a crusade, join forces and hire a regiment of sergeants/knights. Attack a friendly catholic stack with the crusading general's stack (first removing form crusade status of course, and needs some microing to get it to work right). Then in battle, auto_win defender and resolve. Try to retrain battered units in the town they fled to -> cannot retrain due to no units available. End turn. Try again next next -> still cannot retrain. Repeat cycle for 5 turns and still not able.

With old save:
Already have several battered (between 15-45 soldiers (Ghazi/Mutatawwi'a) in a max 48 regiment) in several cities/castles. Cannot retrain. End turn several times and still cannot.