Resetting the Google Account

Try this when you are locked out of your google sync account and do not want to reset the device.

First go to Settings > Applications > Manage applications, and tap the Google Apps entry.
Then tap 'Clear data', and tap 'OK' to confirm.

This will delete your existing Google account details. To enter your new account details go to Settings > Data synchronization > Google and you'll be prompted with the set up dialogue.

Enter your new account details and you're done.

What this won't do for you is move all your mail, contacts and calendar data from the old account to the new one, so in order for all your data to appear on your phone as it did before you'll first need to transfer it between the two accounts.