I understand your cabin fever and the urge to get some cases formed. I went through this phase while waiting for my gun also. I was going to form some ahead of time, except for the last stage but I ended up waiting until I had the gun and it worked out fine. It really doesn't take that much time to neck down a couple hundred cases and having the gun/chamber there is the best way to get the slight crush fit Kirby is talking about.
Just try to maintain the original shoulder angle and you're right about moving the die/bushing, otherwise the angle would get off. The important part is the final fit. After you touch off the fire form load all's well with the world and you have a nice sexy looking case to start working with.
This is a picture of a standard 338 Lapua case on the left, then second is a sample case that is ready for bullet seating, third is a fire form load and on the right is a fire formed case that has been loaded for full power testing. I've only done a couple hundred so far but I've never lost a case. I'll probably do 200-300 more this spring. Sorry, but I don't have pictures of each stage.