by crow » 10/24/2017 12:34 AM
You can, but only during wave releases, or other periods where you're also able to switch out barracks pets entirely. It's basically counted as if you're swapping one pet out for another, even if you're just changing the traits on a pet you already have in the barracks.
Unfortunately, the last wave release was quite a while ago, so right now you wouldn't be able to, no.