Some of us worked really hard to collect breeds. I collect specific breeds for every pet and for those battle pets with 10 breeds; Yep, I started a lot of battles to find the right breed. Some of the rare pets are a bit of work to attain 1 of much less the breed you want. Even more so, when you decide you want a different breed and go back and do it again!
While the idea a mutating battle stone would be a nice catch up mechanic for new pet battlers. It should not be cheap, as it could be really valuable on some of the AH pets that go for lots of gold. (though perhaps it could be made to not work on all tradeable pets)
If it was made to not work on tradeable pets, and it changed to a random breed (excluding the current one) (maybe with the loss of 1-3 levels in the process), I’d say then around 25 to 40 charms sounds about right.
I really don’t like the idea of such a mutating stone working on tradeable pets. It would take quite a bit away from the collecting game. If it did I wouldn’t like to see it for any less than about 150 to 250+ charms for such a stone.
just my opinion. YMMV; while were talking pet charms, there should be SOMETHING useful to spent the old shiny charms on, bandages, treats, or leveling stones even if priced a little higher than they used to be.