true story: this exact scenario happened to me recently with chickensmoothie (found out I had a years-old empty account I had forgotten about like a month after starting to play on a new one)
was real scared at first knowing how I'd be done for if it was like on FR but from browsing the help forums I got the impression the admins there were more understanding, so I decided to risk sending in a support ticket explaining the situation and asking them to close the old acct + let me keep playing on the new one
and within a day I got a nice reply back saying "ok, we closed the old account, thank you for letting us know"! and that was it, no hassle, no lost items. it really can be that simple