There is no reformat necessary as such. An iPhone can only synchronize to a single computer, though, so when you connect your iPhone to the new computer, you'll be told that your iPhone is synched to a different computer and given the choice of erasing the content on the iPhone or transferring iTunes Store purchases to the new iTunes. The best approach is probably to copy the entire contents of the iTunes folder in your Windows system to your Mac. Then your iPhone should still think it's being synched to the same library and be fine. Someone here can point you to instructions on how to move your iTunes library if you need them.