So, from what you wrote, you have an existing site. You've built a new site with GoCentral. You want the domain for your old site on the new one.
If that's correct, then first thing you'll have to do is remove the domain from the old site. I'm not sure what platform you built it on, so you'll have to look up some instructions on it. If it's also a GoCentral, the next set of instructions will help.
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