It's weird to see how Japanese treat the gender "I" as a inherited self gender thing. Rather than the what you were used to say to due social upbringing. But I guess it's a useful shorthand to show how character's self image of gender is changing. Even if it's unrealistic.
Something more realistic would be for character to use masculine version and as time passes be more uncomfortable with that. Either due to social quirks that can bring up. (oho you are a girl that uses ore?, etc), so they switch and find themselves more comfortable, as they are now treated like a normal girl. Or simply because they don't feel like it suits them anymore.
But I guess, if the force that is changing the character is magical in nature, that force could also apply a specific condition on the character. So a "refers to themselves in feminine manner" can be one of the changes applied. *shrug*