So I'm an "old newbie" here, I don't think they had forum titles when I was here before - it looks like you can get them from earning achievements.

But can I not go back and get titles from achievements that I already earned before the forum titles existed? i.e "Only room for one"

, no this is not possible to get titles for legacy Achievements. The only room for one is still possible, if you temporarily loan out all but one of your pets to a friend or friends who have the petslots.

There used to be a link on the Achievements page that you could click to "refresh" your list of titles and retroactively gain titles for achievements you already have, but with the recent revamp I can't find it. It might be worth sending a note to a staff member asking for help.

Oh! That is a good idea . I had presumed since the link was gone it was no longer possible, but I do not actually know this. It is worth a try

Thank you both for the information. Should I ask the staff about this through the ticket system?

yep, that's right.