Linda ~ Welcome to Pet Talk.
I'm sorry, but I don't have any advice about underbites. None of mine have that. Halo has "fangs" that hang down from her top row and stick out of her mouth just slightly. She looks like a little vampire sometimes.

It's probably not a big problem for Snowball. Unless it causes problems with eating. Please keep us posted on what the vet says.

Lord, what's next? Kitty braces?