Is the home directory configured into each end user? If so (and you are not using DB or AD users), you might want to approach things like this:
1) Make a really good backup. The Serv-U Backup KB is a good place to start.
2) Pick out a couple of test users to work with. Test them out before you change anything.
3) Take the server offline. (Don't stop the service, but shut off access from outside the box.)
4) Perform a User Export from your users tab. This will yield a CSV file.
5) Open the CSV file. Find/replace D: with F:(and make other path adjustment as necessary, e.g., "D:\home" -> "F:\users" or whatever)
6) Copy everything from D: to F:
7) Perform a User Import from your users tab using your modified CSV file.
8) Restart the server. Test your users. Upload some fresh content and make sure it goes to F:.