[throwaway20371]

It can work this way (that's how software patches have historically worked) but if you don't test it from the beginning, you will still find the odd case where that added step is broken, even though it seemed like it should have worked. The more you use that method, the more chances for breakage.

If you don't want to repeat the steps from the beginning, you could make a completely separate checklist to be followed on a given system that includes things like "make sure X package is installed", "make sure Y configuration is applied", so that the new checklist accounts for any inconsistencies. This is pretty common anyway as checklists are broken up into discrete purposes and mixed and matched.