When a full length sizing die is set properly, it will move the case shoulder back just a few thousandths, and size down the lower part of the case slightly. You can neck size with the same die by moving it up about 1/8th turn or so and the shoulder will not be moved, but the neck only will be sized.
What I do is set my die to F/L size bumping the shoulder slightly. If I want to neck only size with the same die, I use one of Skip's die shims I bought from Sinclair. When the shim is removed, you can F/L again without changing the die setting.
