Special Cases
Some cats have been known to exchange their life as a warrior to be a medicine cat (for instance; littlecloud, HawkHeart, Jaybae, mudfur, mothwing, or yellowfang) while some have life-altering experiences that may cause them to take that path (such as, Cinderpelt) but this is considered to be very uncommon. Though most medicine cats believe in StarClan, a few rare exceptions (i.e. Mothwing), have become medicine cats despite their lack of belief. Medicine cats usually keep their role until they die, with the exception of Goosefeather, Lafpool, and Runningnose.
However, Leafpool did return to her medicine cat role after the issue of having kits was settled in the very end of The Last Hope, and she was a ThunderClan medicine cat again instead of a warrior along with her former apprentice, Jayfeather, in Bramblestar's Storm. While medicine cats are not permitted to have mates or kits, cats that have previously had and lost a mate may become medicine cats if they choose, even if they still have kits (i.e, MudFur).