Basically, they have been maintaining software for the last few years. Very few of their applications use subs (some of these are thousands of lines long), and none of them use modules.
I would start out by teaching them to refactor. To refactor with confidence, you'll need to teach them about Unit Tests. They can practice writing new code in unit tests, and gain confidence using subs and modules as they go about refactoring. When they come out the other end, they may have gained enough confidence to start writing new stuff.