I have two Сloudflare accounts, I registered a domain on one, I want to link it to another account, but I need to change the nameservers there. I go to the first one, I click on the registrar, I find my domain, I click on the Manager button and I get “404 | We can’t find the page you’re looking for.”
You’d have to transfer the domain to another registrar first, then add the domain to the second account and point it at your registrar towards that account, then you can transfer it back to Cloudflare should you wish so.