can you clarify a bit more? You can add/remove custom domains of your own. If you're talking strictly about the pages.dev subdomain that is assigned to you, you cannot change that without recreating the project
just be careful, you can do that, but you'll have to readd the custom domain to the new project, just changing the dns records aren't enough, and it's going to incur some amount of downtime
even if they offered a managed way to do it, don't see how they would avoid the downtime of resetting up the custom hostnames without some serious magic/overriding of internal stuff