Splitting states is specifically addressed in the section. It just takes a vote of the involve state legislatures and Congress. Moving counties from one state

Splitting states is specifically addressed in the section. It just takes a vote of the involve state legislatures and Congress. Moving counties from one state to another doesn't require Congress be involved as no state is created or destroyed. I assume that legislatures of both states would need to agree to the border change but it isn't something that is addressed.