Dupes are not normally deleted (see forum #3113), only when considered on their own they are low enough quality to warrant deletion. On the other hand, when there's a much better version, flagging the artifacted version for deletion would be good. (This transfers favorites to the parent now, doesn't it? Otherwise it should be left, I think.)
Though I don't know how the mods would feel about a flood of better versions. I'd upload a better version if the pic we have already is significantly worse (i.e. it's from aerisdies or otherwise has visible jpeg artefacts, lower resolution, poor scan quality, etc.), but not if the image is only marginally better (like uploading a png if we have a good quality jpeg).
But then I'm not a mod, so take that with a grain of salt.
Maybe you could post some examples of pics you're planning to upload good versions for.