I just copied the iTunes folder to my mini when I made the move. I have not deauthorized my old computer (yet) since that was not necessary.
I would move first and clean up and merge folders afterwards and keep the old iTunes folder(s) on the laptop as a back up until I was done.
Depending on the size of the hard drive on the mini and the size of your iTunes folder you might consider moving it to an external drive instead. That way you can keep more free space your system disk.
I just moved the entire iTunes-folder and could just continue using iTunes on the new computer where I left off on the old one. Don't deauthorize the old computer before you are finished with it and everything works on the new one. If something goes wrong in the transition the old itunes folder can be valuable... (I still haven't deathorized the old one nearly a year after)
If all your media is contained in the iTunes folder
it is quite simple. If all your media files are not in
the same folder, then open iTunes on the old computer
and in the Files menu go to consolidate files. This will
move all media content into the iTunes folder.
Next, move the entire iTunes folder and place inside
the Music folder inside your user folder. This is
the most common place to put it, but can be put anywhere
(just remember where you put it).
Next, and this is important, you must then open iTunes
while holding down the option key. This will bring up a
dialog box asking you if you want to create a new library
or use an existing one. Select existing one and then in
the file selector that comes up navigate to your iTunes
folder and select the iTunes Library file.
If you did everything right, it should open up and be
just like it was on the old computer, playlists and all.
ive done a daft thing... ive been tidying all my iTunes data and copied it all to a new drive and tidied it up so its just how i want it.
Can i now at this late stage go back to m,y original library, which is still there, waiting to be deleted, and recover the play counts and things like that? Is that stuff in one file like the iTunes library file?
What does the library file hold in it?
Items like play counts, etc. are in:
/Users/[username]/Music/iTunes/iTunes Music Library.xml
It hold all sorts of information. You can open it with text edit, it's just an xml file.
As for restoring what you the old library info, you *may* be able to if you still have the original "iTunes Music Library.xml" before you started your iTunes housekeeping. If you've been modifying the same file all this time, then unless you have a backup of that file, I don't think you can return to the old structure.