Reactivate Old Website for Returning Users

In MCP, change the name of the old website, just like you would during a Domain Name Change. https://support.realgeeks.com/hc/en-us/articles/219137097-Changing-Domain-Names
If they re-signed up with the same domain name, then add a "1" at the end of the name on the new website in MCP.
Keep in mind that the client may want to keep the old domain name or use a new one. Be sure that the desired domain name is applied to the target MCP site profile.
Change the Enabled Flag for both sites to enable the old site and disable the new site.
Follow the Changing Domain Names instruction to adjust the CRM settings.
Verify the Site Owner in the Edit Company page.
If they have approval, then go ahead and push the website live.
If they don't have approval, then their development site will be at their olddomain.realgeeks.com still.

Merge Users if they signed up with a different email address.