A plain-language look at how the recovery works. The exact method is part of the client deliverable.
When the new site launched on a brand-new address, Google saw a website with zero history. None of the ranking built up over the years carried over on its own. All of it stayed behind on the old address, doing nothing.
The longer the two stay disconnected, the more that stranded value fades, while the new site keeps starting from scratch. There is a correct, established way to carry almost all of it across. Done right, it recovers the bulk of what was built; done wrong, it can make the loss permanent. That is the fix this guide is about, and the part worth handing to someone who has done it before.
Here is the shape of the work. The specific method, settings, and the order they have to run in are the deliverable, so they are withheld here.
The core fix is the urgent part. These follow-ups lock in the recovery. The how is part of the engagement.