Actually, I think the newest version of Calibre is much more
friendly to pre-existing libraries. I
believe you can simply tell it "go look here for the files" and it'll populate the program. I upgraded to Win7 a little while ago and just copied my Calibre library over and pointed it to that directory. I haven't noticed any problems yet.
You might want to drop by the Calibre forum (or a mod can move this) and ask Kovid himself.