Hehe, I did exactly the same thing. I emailled the support team and got this:
"Dear <hide my name!>,
Thank you for contacting the Square Enix Support Centre.
We are very sorry to hear that you accidentally registered the wrong account in the Beta test.
We have forwarded the issue to our head office to see if there is a way to transfer the Beta service to your active Legacy account.
We will contact you as soon as we have an update regarding your situation.
We apologise for the inconvenience.
Kind regards,
The Square Enix Support Centre team"
I'll let you know if anything comes of it.