Once you've moved files outside of MC, you -can- update MC with that info... I do this myself, because if I use 'update library' I'll lose all my data as I don't store the info in tags, just the library.
To update MC's filenames you can do a search and replace on the 'filename' field... select all the files you want to update the location of (I'd -test- this with only a couple first,) rightclick and select 'Library Tools\Find and Replace'. Click on 'Select None' then scroll down the list on the left and tick the box of 'Filename'. In the 'Find What' box type the old directory: eg "D:\stuff\music" and type the new directory in the 'Replace' box: eg "X:\otherstuff\newmusiclocation". Then you have to click off the song (either changing view scheme, or just clicking on another song) and MC will update all the files (and since the old files aren't there anymore, it can't move them and just updates to the new location).
You can also use this same method -before- you move the files, as if the files are still at the old location MC will move them itself... have a play around with this before using it on a lot of files, but it should work fine :)