The replies are listing a lot of tangential rules like "major/minor", "typo", "punctuation", "[in]significant change", but all that really matters is: Does your change add, remove, or alter meaning or even tone in your post, even implicitly? If so, somebody may have written a reply that no longer makes complete sense in relation to your post (or they may currently be writing it! "Nobody has probably even read it yet" is not a good thing to assume on an internet where the page remains static until they refresh).

The big thing is: It's hard to imagine all the ways a change can conflict with replies, and the HN crowd is probably more likely than most to "know what they don't know" (i.e. more likely to play it safe).