From your account (click your profile picture in the upper right corner), choose ‘Premium Subscriptions’:
That will get you a list of all your premium subscriptions.
Find your existing (old) premium site in that list, and click the three dots on the right to get a menu of options.
Click ‘Assign to Different Site’. From there you’ll get walked through the process. It’s pretty simple and works well. My recollection is that moving the domain is an option you’ll have during the process.
