Check out Ningyo no Ouji-sama, it is exactly what you are looking for but it only has 2 chapters translated

in chiro star project she kind of gets turned into a boy?? they force her voice to change and stuff but idk, try it out. it doesn't exactly count as transforming, but it's a bit more than just cross dress

InVisible

I'm not sure about this but how about Fujoshi trapped in a Seme's perfect body? It has Yaoi so... don't know if it's your style or not. Anyways the main story is about how a girl saves a old lady from getting hit by a car and in returns she allows the mc to have a wish and the mc wishes to become a boy. 😁