Swap data, profiles, app on two Mac with Migration Assistant
Migration Assistant works best on a cleanly erased Mac.


borrow/buy a drive large enough to back up Mac A
Time Machine Mac A onto drive and then disconnect the drive
Erase Mac A and install new OS
Migrate everything from B to A - connect the two to the same network or use Target Disk Mode
Erase Mac B and instal new OS
Migrate data from the backup drive to Mac B


Swap complete.