[CMake] Why does changing -G not change generators?

[Paul Smith]

We have a situation where people are sometimes wanting to switch between
different generators: from Xcode to Unix Makefiles, or from Visual
Studio 2010 to Visual Studio 2012, etc.

If they have an already-configured workspace then run "cmake -G" with a
different generator, it appears to be a no-op: no error is generated but
no changes are made and the previous generator is still used.

This doesn't seem right to me... shouldn't giving a new generator type
on the command line change the output for the new generator?  I'm using
CMake 3.1.0... maybe this has changed since?

Cheers!