Here is a handy little spreadsheet for calculating the thickness of neck brass after you have resized it for a different diameter bullet.
Several times, I have resized 308 to 243, and have never had any problems with the brass not chambering correctly. But if you're curious, this will put numbers on the amount of change.
For example, if you go from 308 to 7mm-08, you pick up a bit more than .001" in neck brass thickness.
