It depends on how much history and data you have built up with the original files that are on your local disk. If you have data you want to preserve, like ratings, etc, then you'll need to do a longer procedure to tell MC those files have moved.
On the other hand, if you don't care about play count, last played date, and all that stuff, you can just tell MC to forget the old files. I would do it like this:
0. Make a database backup. File > Library > Back Up Library...
1. Go to Audio > Files.
2. The first pane on the top should be Location. Select the location of the files on your local disk that you want to *remove*. Verify that you have the files you want to eliminate from the Library.
3. Select delete (from the right click menu is the fastest). Choose the option to remove the files from MC's database ONLY. This way they don't get deleted from disk also. You can manually remove them from disk later if you want to.
That's the easy way, but again, you'll lose all of your play history and stuff. If you need the longer way, I can try to type that up too.
Brian.