As far as alleviating fears goes -- and I know this is stating the obvious -- using a version control system would be 2nd on the list after a good test suite, as Mutant has so eloquently pointed out.
If I really botch the job, or if I discover half way through that I wasn't as clever as I thought I was, I know that I can always go back.
Actually I'd put RCS and test suit in the other order - RCS first. Going back in time is much harder that tacking on a test suit at some point in the future. Indeed, with an RCS you can retrospectively write the test suit against the original code even after the code refactoring is notionally complete.