Greetings!

Exactly. Because the 80 was built and has the OS on it, it can act as either master or slave. If you:

a) boot with your 80 as master and your original 30s as slave
b) copy the fids from your 30s to the 80
c) reconnect your original 30m as master, 80 as slave
d) reboot, resync

you should get exactly what you want. Better, because your original database files will be intact on the original master drive, and you should not (in theory) even have to rebuild the database. Best to do so anyway, just in case.

Edit: for the sake of sanity, you may want to delete the following files from the 80GB drive. This way, you do not have an old / invalid copy of the database on /drive1...

/drive1/var : config.ini, database, playlists, tags
/drive1/var itself