[Makinus]

The only way to advance from one form to another is in battles? Even so it could work... we could justify Daughter -> Mother as battle experience as the sprites are not too different... but Mother -> Crone is harder to justify (maybe fighting causes early aging to Avalon witches? could be even thematic with the curse that shows up in LA Man).

Another idea would be a ritual that transforms one unit in another (Orc Empire does something like this with the Mawgut), but i´m not certain yet in how to restrict the ritual to only a single type of unit....

Thanks for the answer... i´ll think a little more and see if i find another way....