Did you create this new form in the FormsCentral "desktop" application that comes with Acrobat Pro XI? If that is the case, and you were not signed in, your form is "local" and not connected yet to any online account, you could simply sign in with your paid account and then "Move Form Online" which can be done from the My Forms tab, or if you open the form from the "Collect and Manage Responses Online" tab.
If you created it signed into a different account and wish to "move" it to your paid account you have a few choices. You can "share" the form with the other account, and in that account you could "Duplicate" the form so it is owned by that account (and could then remove the shared form to avoid confusion). Instructions for sharing: http://forums.adobe.com/docs/DOC-2462
Another method is to "Export Design File" from the "File" menu on the Design tab when the form is open, you would then go to the paid account and "Import Design File" (from the File menu while viewing "My Forms").