I know someone who switched into bya years ago. I think the only way to go about it, it's too so a lot of research, and find out if there's a semi where someone ended up leaving, or didn't end up coming.
The seminary would need to be convinced that the girl is leaving for very valid reasons it won't be easy. If she's looking for something BY you can try one of the new sems like Bnos Shoshana or Havineini. Maybe Kesser Chaya. If she's looking for something more modern then it might be easier.
If it's just a case that the girl and the seminary were not a good match, it's fine to go to the school where you think she'd do better and ask if they will take her. Some of the decision will be based on the relationship between the two schools, and how open everyone is about the situation.