I have a 90 coming out of my wall in my utility room to turn the copper down the wall. I would like to swing it up as I have a taller softener. Do I need to disassemble and start over or might I have success simply heating the joint and swinging it up?
Most likely, you'll need to cut the pipe and rework it with new fittings. If you heat the joint hot enough to melt the solder, you'd want to pull it apart to clean the joint, apply new flux, and solder it again. But in my experience, it requires too much force to pull the pipes apart, and when you pull with a pair of pliers, you end up crushing the now soft pipe.
You can indeed desolder fittings. However, as BMitch points out, you'll have to remove the fitting, clean it, and solder it in again.
If you heat the solder up enough, the fitting should slip right off. If it doesn't come off easily, it's not hot enough. Try to spread the heat around, so you heat the joint evenly.
You'll want to use a pair of pliers, to remove the hot fitting. Do not touch it with your hand.