Leaving the old server around for restores is an excellent option that many people have done. As long as your old server isn't going end-of-service life, this seems to work well if you don't have very length retentions. If, say, your maximum retention is 1 year, then by the end of next year you won't have to worry about it. I don't know about you, but restores from those longer retention tapes are very rare for us.
If you need to import the tape into your new environment, the procedure is straight-forward and documented. It's not quick though - for a full LTO-3 tape, I've heard that it can take a day to do a full phase 1 and phase 2 import. This might not be an issue if you have the time to wait for the import to complete or if you have few tapes and excess tape capacity.
Some people also decide that if a critical restore comes through for tapes in the old environment, they can always reconnect the library back to the original server for restores.
You've got a few options and it depends on how big the environment is, how many tapes you have and how long those retentions are, and how bad you want to do a cross-platform migration.
