You can flip the case upside down, the gears won't fall out. You will still be able to slide the shaft back in afterwards. You can slide a dowel in the other side to help lift the gears and align things.
It's pretty simple, biggest thing is correctly torquing the nut afterwards.
The transfer case rebuild manual tells you to measure the turning torque. Most people don't have the correct tools for that. Just make sure all your shafts turn easily afterwards. If you can't turn the shafts by hand something is wrong.