iTunes Brain Surgery
Problem:
You have over 15,000 mp3s that exist on a remote server, and you want to relocate them to your local machine while keeping your iTunes Library information (mostly) intact.
Solution:
After moving the files to your local machine, iTunes will stubbornly look for them on the remote host. If said host no longer has those files, iTunes will ask to locate each individual file (which is bad.)
Export your iTunes Library from the File..Export menu. I recommend doing this twice, just in case.