First, I'd make a copy of the file for testing.
The text frames must have some sort of Object style applied, even if it's only the default Basic Text Frame. You should be able to redefine the styel to alter the number of columns.
Not necessarily, but it would be ideal.
If the layout conforms to the master page you could try changing the
master to multiple columns. Either way, ABSOLUTELY make a copy of the file.
By "must" I mean that they probably have either the default object style or a new one assigned. It's pretty unsual for users to set a text frame to No Obejct Style and it won't get that way by accident.
I don't think your master page idea is going to work, either. Changing the column guides on the master won't affect the number of columns in any existing frames. Changing the number of columns in a master text frame might work, but I think the object style is a better bet -- no guarantee there are any master frames to work with.
Good points. I should probably stop thinking out loud.