This is a hypothetical question that lives in a sci-fi like future.
Transgender people currently don't have access to an awful lot of options to change sex characteristics. Early intervention prevents a lot of secondary sex characteristic development. Other than that, there's hormonal and surgical interventions that do not change a lot of their underlying anatomy and genetic/epigenetic sex traits.
In the future, let's imagine that changing sex is an easy medical option available to anyone. Transgender people can quite easily change their anatomy, and even their genetics. They do this while still remaining the same human, with the same prior experiences.
Then the question becomes less 'have they changed sex' but rather 'ought society to allow it'?
What are your thoughts?