Rivets are fast if you do preventive maintenance before the first cutting. I use my large vise to blast (shear) the rivets. Park the knife in the vise with the bar laying across the jaws. Blast with hammer. 2 points if you score both rivets with one hit.
However, that being said, my mower stores a 2nd bar in the frame cross tube. When a skip starts in the field, I either change the knife on the fly if cutting in daylight, or change the bar if dusk (mosquito issue). This means keeping your header, shoes and hold-downs straight and level so the bar slides out easily.
I changed over both bars one day five years back. Its not that hard but if the holes are oblonged from loose rivets, the bolt splines won't bite. Use a peen hammer to tighten them. I use a battery impact to get the nuts close. but torque by hand, else the impact might shear a few off (PITA).