JohnGV, I am going to have to
entirely disagree with you on the squwking
If you are talking to an ATCU and have been given a transponder code you must NOT change unless you are told to! What happens if there are two emergencies? Both with the same emergency code of 7700?
If you have transmitted a distress message over the VHF frequency you are currently working then as Backpacker said, they know the area you are in, they know who you are, they know what height you are at if you've been using Mode Charlie and they can track you. They will sort out SAR if needed, thats what they are there for, they have procedures for it. There is no point in squawking 7700 as you've spoken to them, they know your registration as that is part of the mayday call and they will have you identified on radar with a tag so leave it at that until you're told to change the code.
I would agree with you if you weren't in controlled airspace, didn't have an assigned code and/or were not in contact with an ATCU. Then squawk 7700. But otherwise leave the code as it is until told to change.
C250