How can change from sync'ing an iPhone (or iPad) to a PC to sync'ing to a Mac? iTunes on PC was used before to sync and update my iPhone 3, iPhone 4, iPad, iPad 2, but recently I got an iMac 27 and started sync'ing the iPhone 4S using the iMac.  But when I plug in the iPhone 4 (the old one) into the iMac, then I can't drag Music, Movies, or iTunes Univ videos into this old iPhone. (in iTunes 10.5 on the iMac, I can't drag and drop any Music to the iPhone 4 under DEVICES)

What is a proper way to start sync'ing with the iMac if it was sync'ing with the PC before?  I just want the iPhone 4 "as is", but sync'ing to the iMac instead of the PC.  (want to keep iOS 4.3.5 because I might do some development work and want to test on iOS 4.3.5)  Are there other precautions to take note of when changing which machine to sync to?
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.