You won't be able to transfer back for some time I'm afraid. Once you are placed on a Legacy server, you are "locked" into it. Right now, only "Standard" servers are allowed to transfer. For Example; I transfer Off Diabolos and switch to Gilgamesh. I can transfer again as both servers are "Standard". If I transfer off Diabolos to Hyperion (coincidentally, I have a Legacy character I haven't bothered to log into since launch on Hyperion) I'm "locked" onto that server until Square opens legacy transfers.

Legacy transfers have not been initiated yet though I believe that they announced they should be able to transfer off 6-12 months of FFXIV going live.

Wish I had better news on this one.