I say give it a chance, Midnight. Start writing the new ms, not worrying about the age level. Just write what feels appropriate for the story. Then step back and evaluate.
Don't limit yourself. Tell the story (stories) you WANT to tell and then decide.
This. If you don't make it intentionally either way, then you won't be limiting yourself. I never intended to write Adult, but I when I realized the project wasn't going to work as YA, I changed the audience instead.