I had a situation once where i needed to cut and past all events from a sheet to another . The original sheet had several global variables. When i tried to cut and paste them all in the new sheet(including all global vars) C2 wouldn't allow me saying something about the globals. It should work shouldn't it ? It appears to be a bug.
@Kiyoshi: I tested the cut/paste of global vars in the same project from an event sheet to another (and only global vars) in r76, recently in my current entry for the comp.
All went smoothly, so I guess either it was buggy or not fully implemented that one time you tried.
I believe it's now working.