Fictional characters also have a bunch of naming rules that real people don't. Quite importantly, unless you're GRRM they should not have the same or similar name to another character in your story (i.e. what you're saying, one name per person and one person per name). Else shit's just confusing, as knows anyone who tried to learn anything about Elden Ring's lore.

Names also should match the character's personality. John's John because he's meant to be pathetically average and John is the most average white american name there is. Naming him Bartholomew or Rico would not have worked for that particular character.

I would therefore posit that if John as a fictional character did transition, Jane might be a better fit due to being more common while still being familiar enough not to be confusing. Or perhaps Jess which retains an alliteration but sounds even more "basic" then Jane IMO.