Are you trying to change the email address associated with your Barnes and Noble account?
First, you will need to unregister the Nook in order to change the email address...
Go to Settings then Device then Register. Select Unregister
Next, we will need to change your email address
Click here: https://www.barnesandnoble.com/account/
Scroll down and select Manage account settings. You may be prompted to sign into your account. From there, you can change the email address
After you change your email address, go back to the Nook and go into Settings then Device then Register. Use the new email address to register
No. The information will be available when you register your account back using the new email address
If you deregistered and followed the steps to change the email address as stated above, none of your data would be deleted. I am sorry to hear that you had issues.