Q: If my iPod is formatted for Windows and then I switch to a Mac, can I use the old music files themselves if I transfer them to the Mac and the iPod itself has been reformatted, or do I have to re-rip that music again on the Mac?
A: You’ll be able to use your music files; they don’t care what kind of iPod they’re on. You can copy the files from your Windows PC to your Mac, then add them to the Mac’s iTunes library, and sync your music to your iPod.
Note that only 3G iPods or later can work on both Macs and PCs; if you have an older iPod, you won’t be able to use it on the Mac without hassles. If you have a 3G iPod or later, simply connect it to your Mac, then run the iPod Software Updater to reformat the iPod and install the appropriate software so it can run on your Mac.
(This works in the other direction as well.)