... it seemed prudent to ask how easy it is to pop the tops on all three drones and replace their GPS batteries?
I might be wrong, but I think you are about to do a whole lot of work that is not going to produce any benefit at all.
Dead GPS batteries are a possibility when GPS will not acquire quickly. But if GPS will not acquire at all, it is far more likely to be the flight control module.
A couple of ways to tell the difference:
1. Leave the drone powered on, with a clear view of the sky, for about an hour or so. If GPS does not acquire, the problem is not the battery. IF GPS does acquire, but also requires an extended acquisition time the next time, then the battery probably just needs to be charged. The battery is charged by turning on the drone and just letting it sit. Don't put the props on and do not start it. Just let it sit for a few hours. How long depends on how dead the battery is. A few hours may be enough. Worst case, it could take a couple of days.
2. Connect to the GUI. If the YAW Icon begins to rotate, it's a dead giveaway the Flight controller is bad. There are other clues.
@h-elsner listed 5 main symptoms in the "GPS Acquiring problem.PDF" linked in this post:
Typhoon H IMU related problems