Now, say you simply begin stipulating "by 'I' I mean...", etc, until you've consistently rejiggered the whole conceptual scheme to get the kind of outcome the uploader wants. Could you really do this without serious consequences for basic notions of welfare, value, etc?

No, and that is the point. There are serious drawbacks of the usual notions of welfare, at least in the high-tech future we are discussing, and they need serious correcting. Although, as I mentioned earlier, coining new words for the new concepts would probably facilitate communication better, especially when revisionaries and conservatives converse. So maybe "Yi" could be the pronoun for "miy" branching future, in which Yi go to Mars as well as staying home, to be merged later. There is no contradiction, either: my welfare is what I thought I cared about in a certain constellation of cares, but now Yi realize that was a mistake. Misconceptions of what we truly desire or like are, of course, par for the course for human beings; and so are corrections of those conceptions.