Personally, I would just ignore any changes in the app prior to check in.
But then if they still downgrade you at the check in then demand the original suite and show them the NUA confirmation.
In my experience, the FD either upgrade youback to your original suite or make all available lame excuse under the sun to make you keep your downgraded suite.
As for email proof, everytime I have issue with my reservations, whether its SNA issue or Your24 or downgraded from the room type you booked with cash, the FD always ask for one.
I guess it was the easiest way to deny anything if the guest somehow failed to show the proof on their end regardless the history shown on their own system.