Nichole,
Yes the underbite can be fixed. Liberty, my yearling, was seen again by her equine dentist who said Liberty has a slight underbite so she had to remove hooks so her teeth could align correctly.
She will return in 6 months to inspect her bite again. Liberty was not born with the underbite and did not develope it until sometime between 7 months old and 15 months old. So I was so glad I had scheduled the dentist to come out for routine check up. Liberty is a pet for me but I still want her to have a correct bite to avoid any eating disorders.