Old shock is out, new one is in. All I had to do is follow the recommendations above: pound the bolt back in, get the shock supported so it didn't travel with the bolt, and support it from underneath a bit. The spring was at such an angle there couldn't have been much weight on it, but I still wrapped it with a towel, strapped it in, and packed the area in with that grungy mattress and old pillows. By the time I got the bottom half of the shock out, the spring simply hung in place, so there could not have been much weight on it.
Putting the new shock in wasn't bad, either. Compressing the spring is the most labor intensive bit, since I was taking my sweet time and being very careful to compress both sides at the same time.
Pics to follow. I want to swap the other side first.