I have had airplanes placed into the experimental category in Canada while I have been flight testing a change to the airplane. This was temporary, and in all but one case, the airplane was removed from this category, and approved as fully certified after the testing (experimentation) was competed. I'm not sure about the FAA system, but in Canada, "experimental" and "amateur built" are two very different categories for different purposes. I have seen N registered airplanes, which appeared to be solely amateur built, with "experimental" placarded. I'm not sure if these planes were non certified types being used for flight experimentation, or just "fun" - the "experimental" aspect of the their flying was not evident. Some non certified types (I'm thinking of Burt Rutan's space lift aircraft are obviously truly experimental, and due operation in that category). But all of that is outside the scope of recreational use of an airplane.
With the exception of the Canadian Owner Maintenance category (with its own specific purpose, eligible types, and limitations), I'm unaware of a method to take a previously certified airplane into an amateur built category, without actually and evidently actually building 51% of the airframe. That's not just bolting 51% of the parts of certified airplanes together, but cutting and riveting (or laying up and bonding) 51% of the airplane. I do know a fellow who built up a former Cessna 175 as a Canadian amateur built airplane, but he actually did spend many years building more than 51% of it from raw materials. It was a beautiful job, which took him many years as a labour of love.
Even an amateur built airplane has some burden, if there is a desire to replace a data plate. My boss once bought a beautifully built Thorpe T-18, with the commitment that it never again be flown. Though he did intend to honour that, and sold off the engine and prop, it beckoned him badly! He bought a set of plans and some parts (like a new canopy), and with Transport Canada inspector agreement, and oversight, I built him a new Thorpe T-18 out of some of the parts of the old one, as an entirely new amateur built airplane. In that way, the data plate, and thus liability of the one he had purchased never did fly again, even though quite a bit of the airplane did.