This may be a completely silly question, but if a man had a .243 and would rather it be a 6mm, is it as simple as running a chamber reamer in it, or is the difference in twist a problem?

I understand this is a low speed custom question, so I appreciate your indulgence.
CT

"The number one problem with America is, a whole lot of people need shot, and nobody is shooting them."
-Master Chief Hershel Davis