To change the email address associated with your licenses, you will first need to create a Redgate ID for the new email address you'd like the licenses changed over to.
When you've created the Redgate ID and logged into the customer portal, you can request ownership of the licenses in the customer portal by clicking add existing license< at the top of the serial numbers page. This will pull across the licenses from your old account.
Once you're the owner, you'll be able to see the licenses in your account and manage and distribute them as you wish.
For more information on managing licenses, please visit this article: