Change in Link Behavior - Excel 2000 to 2003

I recently upgraded from Excel 2000 to Excel 2003.

One of the spreadsheets I've worked with using Excel 2000 has many links to
other spreadsheets, all of which are created based on a mapped drive letter
(S, which is mapped to my Public folder. After opening the spreadsheet in
Excel 2003 many (but not all) of the links have changed to the UNC
(\\servername\public\...) to replace the S: in the link. I can't seem to
figure out why this would have happened, and I also find that the links that
still use the drive letter do calculate properly while the links that are
now based on the UNC do not update, even though the UNC path is correct.

Is this a change to the way that Excel 2003 handles links?
Is there any way to have them revert back to using the mapped drive letter.

Another piece of information, I did make one other change at the same time I
upgraded Excel. I changed my "My Documents" target to point to the same
Public folder that S: refers to. I don't see how this would affect
anything, but it's the other change that I made so I thought I put it out
there.

Thanks very much for any help you can offer.
--David