This would be a standard, checkable box!
I have made the mistake (and I'm certain others have as well) of buying a gorgeous lion to sex change, as it would be perfect for my breeding project! Too bad, it patrolled. While I check much closer now, that would make my life easier.

It would also make finding an heir easier, as I have to search for males, and then I have to put myself through difficult constraints in order to find a good female that MIGHT be sex-changeable, and I might get my hopes up with a lion that would be a perfect heir if I sex-change, as it is a 2 year old female, but guess what. It hunted. Too bad.

Another possible thing that can be added along with it is a little tag that shows up next to the lion in the trade listing that says "still sex-changeable", just like the "older lion" "past breeding age" etc