I'm in the same boat (er, server) as you are, wanting a friend to play with me. We have no reason to believe that characters on new servers will be restricted from transferring to the Legacy servers, in theory. But I think the problem may be with this little caveat:

Quote Originally Posted by World Transfer Service
Due to fluctuations in World populations, certain Worlds may not be available for transfer.
Excalibur is unfortunately one of the more crowded servers, so I would not be too surprised if it was unavailable for at least some time. But who knows? Personally, I am considering transferring my character to a less crowded server to play with my friend, either a JP one or perhaps Sargatanas as it seems the least packed of the NA Legacy servers. I'm sure Excalibur will be available for transfer at some point, though, it is just hard to tell when since only SE knows just how crowded it really is.