How to change file path in iTunes Library?

Hi,

For some reason a ****** NAS drive messed up my iTunes Media folder. I had a backup and managed to copy all files to an external drive. But all my files in iTunes won't play because they're still pointing to this non-existent NAS drive.

I changed the path in Prefs -> Advanced but it seems this is only for newly added files. My existing files keep referring to the old location. Consolidating is not an option either because that means copying from the old location (doesn't exist) to the new one.

I could delete my library and re-import my iTunes Media folder but then I loose all my metadata.

How can I tell iTunes to look for my existing tracks on the new external USB drive?