To CHANGE your forums username, you should be able to go to your account page, enter both your CURRENT and NEW names/passwords and have things update on the forums. I don't know if this keeps your old settings (signature, posts, etc) but I assume so. If you end up with 2 accounts, this will only work for the account linked to your account.
It looks like all NEW accounts will require an active subscription. Interesting. I wonder if my old account will get deleted or slide by as a semi-permanent account like on AO forums. DON'T TELL
I'm sure plenty of folks ended up in this boat. The first 1,000 people who sign up are supposed to receive a custom title, "An Initio" (their translation: 'from the beginning'), as per their Facebook page.
They appear to have preserved some of the old forums, unsure if they kept them all.
PS. The forums are nearly completely useless to my way of using forums ... there appears to be no "Latest Posts" / "New Posts" OR quick search options. I found Advanced Search but it does not allow for searching with no terms (ie, still no "New Posts" ability).
