While it sounds like most are moving to iCloud pretty smoothly, it's always a good idea to create a backup of your data before things like this. Here's how to backup your contacts and calendars. Open Address Book, then choose File > Export > Address Book Archive. And then open iCal, then choose File > Export > iCal Archive. In both situations, just export to the desktop. Then, once you've confirmed that your migration went seamlessly, you can move them to the trash.