The law also mentions the person who for time being is the owner of the aircraft. I don't think that you can change owner in mid-flight!
Why on earth not? It's standard practice to do so in many cases -- ISTR a certain company not a million miles from Seattle flying its aircraft over the Atlantic and selling them while airborne to avoid taxes that would be levied if they were in the state of WA at the time of sale!
Don't get me wrong, like you and llanfairpg, I wouldn't fly with another qualified pilot without having the who's-in-command conversation before getting to the aircraft. But the reason there are these awkward cases after the accident is that the law doesn't
require designation of a commander pre-flight.