In iTunes, with the iPad connected, select it, go to the Info page and make sure you've turned off any Calendar, Mail, or Contact syncing. This is redundant and cause problems when you are using iCloud.
In iCal and Address Book go to the File menu and Export your information. If something goes wrong you can use this to recover anything you lose.
Open your iCal app's Preferences and make sure your account is now set for iCloud.
On the iPad go to Settings > iCloud and make sure you are signed in and Calendar is on.