As this is a discussion board, happy to share my readings this evening, although I am not a legal scholar.
Will send my bank details for my consultation fee in the next post ;)
I've been reading R.K. v Hungary, the Jun 2023 judgment referred to by PP as the most recent relevant ECtHR decision.
Summary: Before 2020, Hungary had a process for changing sex on birth records, but it was confusing, inconsistent and unofficial (heavily dependent on whichever official you happened to deal with). In 2018, a trans man (I assume quite young as the case refers to distress at school) applied to change her name and sex marker from F to M.
In May 2020, the Hungarian government brought in new legislation specifying that (a) sex at birth could not be changed, and (b) this new rule was retroactive and applied to any current/pending applications. The applicant's case was still pending at the time.
Earlier domestic courts as well as the ECtHR agreed that (b) was invalid and therefore the applicant was bound by the old system, and the case would judge Hungary's gender recognition system "in force at the material time" i.e. 2018-2020.
Result: The meat of the case, and the reason an Article 8 violation was found, seems to be the inconsistency/uncertainty of the pre-2020 process. The court concluded
(para 76) ...the legal framework in force at the material time... did not provide "quick, transparent and accessible procedures" for the examination of a request to change the registered sex of transgender people on birth certificates.
This seems to be much more about having fast decisions, and avoiding protracted and painful sterilisation/medical requirements etc. There's nothing faster than simply telling everyone "no"!
Concurring and dissenting judgements: Scroll down to the bottom of the link to read them, I really advise it for getting ideas on how people are thinking this could be attacked.
Judge Hüseynov notes the wording of "for the EXAMINATION OF A REQUEST TO CHANGE" rather than "for CHANGING" in para 76 and is annoyed at it for leaving the door open.
Judge Wojtyczek says that "in my view, the Convention does not guarantee the right to obtain a change of the registered sex on birth certificates and the parties can choose how to regulate this domain", and links to the much longer dissents in A.P., Garçon and Nicot v France and X v FYROM, which I will have a look at next :) Skimming through them, I see some familiar GC arguments and they seem to boil down to, this is controversial and there should be a wide scope for states to make their own calls.