I did something very similar to this yesterday when I replaced a 20GB with a 40GB....
Ran disk builder on the new HDD
Set the new HDD to slave alongside the drive it was to replace
Copied the fids dir from to the new (Drive upgrade Guide step 7)
Set replacement as master
Synch with Emplode afterwards to rebuild db*.
I found it quite easy and the copying took about 3.5 hours for 20GB, I didn't retain playcounts or anything else, just the music.
* The Emplode synch took so long to show progress I thought something had gone wrong