First let me say welcome to the community!
As for changing the domains on the email, what sort of email plan was this for? Workspace, cPanel, or Office 365? Knowing a little more about the type of email plan will help other members provide a better suggestion on how you should be able to change out the domain name.
My one tip for any instance is to save the current messages to your desktop using an email client to save file copies for any important messages you don't want to lose. Just in case you have to delete the existing email address in order to setup a new one using the new domain you want to change the plan to.
With a Workspace email, the only way to change the domain is to cancel the existing address so that the account credit is freed for setup towards the new domain. This is where my previous suggestion to save any important emails you wanted to keep to your desktop comes in handy.