Thankful for the kind neighbor with a cordless drill who made easy work of changing out my car tags. Frozen screws were no match for the power tool. 
For the moment the same screws were reused. But when I can get to the hardware store within a few weeks I will get the type of screws and such that all of you have so kindly recommended and swap them out. I also want to wash and wax the trunk lid area under the plate; it needs it.
The front plate screws into the plastic bumper so I could do those by hand but had to wait so I had the matching new plates on the car.
As @Nevada_545 notes, the rear plate screws into the trunk lid. There was no rust. But mercy those two screws were locked tight.
I actually broke my dad’s largest Phillips screwdriver this morning trying to get those two screws out. The handle now loosely spins around the metal shaft. Oh well, it’s about as old as I am, 65, so I suppose it has earned its retirement. But, darn it, that was my late dad’s tool I broke.
Anyway, problem now solved. And thanks to all of you I know what to do going forward! Thank you, again, everyone!