Yes, it is possible to fit a '81 seat to a '80 bike. You may or may not need to swap out subframes, I'm not sure. I do know you will need the subframe if you want to fit the front tool box, as the top mounts are of a different design*, and the seat pan follows a different contour on it's sealing of that box.
Then there is the mounting of the glove box onto the rear fender/subframe.
The '81 glove box is of a much improved design over the '80, BTW.
If I ever put a dual seat back on my '79, it will be a '81 type.
*The later mount design gave much needed additional room for the r/r of the battery, BTW.