Two thing to be very aware of here!
The glass is heavy, if unhooked from the scissors assembly, it can fall down the tracks and break an arm if inside the door working.
Safety first, if leaving glass in door, use a chunk of 2X2 to wedge it up and prevent it from coming down.
Also, the scissors are spring loaded to compensate for the weight of the glass.
If you have it out and are removing motor from assembly, I drill a hole and put a bolt thru the motor frame and an arm of the scissors, as a safety, to prevent it from unwinding. I also lock a big pair of lock pliers on it for the same reason. Ask me how I know this...
And for the non-purests, I drill a hole in the door inner panel right above the one motor bolt it covers, which allows me to remove the motor without removing the scissor assembly. Saves a lot of time and no one will ever know...