You call upon primal energy to transform yourself into a Medium animal battle form. When you first cast this spell, choose ape, bear, bull, canine, cat, deer, frog, shark, or snake. You can decide the specific type of animal (such as lion or snow leopard for cat), but this has no effect on the form’s Size or statistics. While in this form, you gain the animal trait. You can Dismiss the spell.
The inclusion of the word "first" makes it seem like you pick one of the forms the first time you ever cast the spell and then it can never be changed. None of the other Form spells have this word, so I suspect that it's just a vestigial leftover. But not being able to use other forms would be a pretty big nerf, so I wanted to see what other people thought.
I would bet that it's an unintentional inclusion of the word that was probably rooted in not being able to change forms while the spell continues.
I say that because there's no reasoning that supports animal form being restricted to a singular form that doesn't also apply to the other form spells that don't have the same implied restriction.