How to copy projects to a paid plan (or reassign a paid plan to another projects)

Hi,

I think that I have finally found an (almost) air tight method to safely divide testing from production. But I just have one issue, perhaps I’m overlooking something:

Basically I have 2 paid plans, one for production and one for testing/sandbox. But my testing is perfected, so I want to use that as the new production. This i can do by reallocating the DNS to the sandbox.

But now I want the outdated production version to take the role of Sandbox, so I need to copy the project of the new production to the old production project. BUT HOW?

Or alternatively: When I duplicate my new Production project, how do I reassing the paid plan to that duplicate? (so I can then delete the old production project) ?

Thanks!