We are [slowly] migrating to using managed AppleIDs. BEFORE you switch away from your old one:

• Fire up Notability
• Switch to Settings
• Select iCloud
• TURN IT OFF
• Select “Keep on my iPad”
• Switch to Autobackup.
• Make ABSOLUTELY certain your files are backing up to Google Drive
  • If they aren’t, set that up RIGHT NOW
• Swipe out of Notability
• Settings –> Sign into your Managed AppleID.

This will keep you from losing all of your Notability notes when you switch.