I have just under 100 once fired 243 Win brass, fired in the regular 243 chamber in my model 12. My understanding is that you form the brass by firing it in the improved chamber. There would be a slight drop in velocity, but they should be accurate, so just live with it until one has worked through the initial batch of shells.

I also have another 200 brand new never fired shells and there is more work needed on those to trim them all to the same length, do the initial sizing operation, size the neck with my collet die and sort them by weight before I am ready to do any loading with them. I just can't see getting rid of my once fired brass just because I got a new chamber ?