Alright, so as this issue is still not solved, I attempted to use a Retrain Token (as the description of the item states that Gold members can switch Archetype characters to Freeform and vice versa). It didn't work - no option to change the character popped up on the character selection screen. That's when I noticed…
I have received a response from the second ticket asking me not to make duplicate tickets about the same issue, but they DID re-open the first ticket, so I guess it's a mixed bag.
Hello! I did contact support already, they're the ones who told me to post here. They said this was likely a bug and that they wanted me to post to be able to see if other people are having this issue.