I think our existing editing policies cover this already. If you see a post that can be improved, edit to do so and leave an informative edit summary. If it's a new user I usually also leave a comment, where I have more room (and can add links) and which the user is more likely to see. This allows me to educate the user about the issue (while also welcoming him to the site etc).
People already edit to clarify or translate jargon, to add links, to translate quoted Hebrew, and other reasons that are about presentation more than content. Editing the tetragramaton to avoid the sheimot problem seems similar to me -- go ahead and do it, and explain why.
I'm talking here about the tetragramaton in Hebrew. The farther you get from that, the more cautious you should be. We know that some people write "God" and others write "G-d", including on this site; editing to change that feels more like imposing one's personal style on an author, which you shouldn't do.
As with other edits, if the author disagrees with your edit and rolls it back, don't get into a tug of war over it. Find a way to discuss it, or bring the case to meta. We don't have a history of moderator-enforced de-tetragramatonifying, and I'm not suggesting that we should enforce a policy beyond our usual conventions. I am assuming that most people, upon seeing an explanation of this kind of edit, would accept it -- that people cut/paste text without noticing, or don't know about the sheimot issue.