How do I move my domain from one portfolio to another?

You can have up to 5 portfolio sites but only 1 can have a custom domain name. 

See links below on how to disconnect & connect a custom domain name.

• https://help.myportfolio.com/hc/articles/8799370199579-Disconnect-a-custom-domain
• https://help.myportfolio.com/hc/articles/360040203193-Connecting-your-custom-domain-with-Domain-Connect

your portfolio is tied to your adobe id.

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in the future, to find the best place to post your message, use the list here, https://community.adobe.com/

p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.

<"moved from using the community">

Sorry, No forum here... Try the several Portfolio links inside this link
-https://community.adobe.com/t5/creative-cloud-services-discussions/getting-started-with-adobe-portfolio/td-p/14151446
-migrate to a new page https://help.myportfolio.com/hc/en-us/articles/10872890414235-Migrate-Adobe-Portfolio-sites-to-another-account-Overview
More links inside the next message
-general help, including the Portfolio team's official help
-using a custom domain
https://community.adobe.com/t5/download-install-discussions/is-portfolio-still-being-developed/td-p/11583268