I also made this same mistake, namely due to excitement about getting my old character back up and there being zero warnings of what's about to happen!

Support have basically told me I don't have a leg to stand on, which I quote:

"Dear Chris,

We appreciate your frustration in this matter but as mentioned before we are unable to transfer the registration code to your other PlayOnline account.

If you want to escalate this further you can certainly do so by contacting our Head Office on email address sel-id@square-enix.com

We apologize again for any inconvenience caused.

Kind regards
SQUARE ENIX Support Centre"

I asked for a simple explanation as to why nobody is able to administer/alter game codes, to which I was not gifted with a response.

To be fair, I hold my hands up I clicked the wrong button. The system should definitely not be so unforgiving though, imho.

I've emailed head office to see if they actually care enough to keep me as a paying subscriber, as I want to do the additional levels and areas, etc.