Tom Young wrote:

Yes, but just on the fly like that? I assumed that the builder sets it when
he originally certifies the airplane and has to recertify if he wants to
change it later. Am I wrong about that?

Yes, you are wrong. Of course, the gross weight is initially set
prior to any test flying that could be used to reasonably verify if
the aircraft is capable of it.

Subsequent changes only require "notification", not recertification.