I normally only edit an already-published piece for minor grammatical or punctuation errors.

One exception is if I happen upon new information or something to reconsider, such as in the case of a recent article I wrote where one of my earliest readers (following its publication) brought a factual discrepancy to my attention. And even then, when I went in to insert a new paragraph as sort of an "addendum," I made sure to preface it with the words "ON-EDIT" (so readers would know that I went in and added it, after the fact, based on reader feedback). Obviously, it's ideal to minimize this practice as much as possible.