How can change from sync'ing an iPhone (or iPad) to a PC to sync'ing to a Mac?
You can only sync a device to one computer, however if you can locate the backup folder, these contain lots of files which all need to be moved across to the other computer and it thinks it has always backed up this device. Locate it on your PC depending on the OS below.


Windows XP: %AppData%\Apple Computer\MobileSync\Backup
Windows Vista: %AppData%\Roaming\Apple Computer\MobileSync\Backup
Windows 7: %AppData%\Roaming\Apple Computer\MobileSync\Backup


and transfer this file to your Mac:


Mac OS: ~/Library/Application Support/MobileSync/Backup


You will need to authorise your new iTunes account and transfer all your music and apps.