If they have the same value, but just became worse at fulfilling them, then it’s more something like “epistemic drift”; although I would probably discourage using that term.
On the other end, if they started caring more about homeless people intrinsically for some reason, then it would be a value drift. But they wouldn’t be “less effective”, they would, presumably, be as effective, but just at a different goal.
I agree that someone can become less effective at acting on a set of values without changing what their values are. The inverse can also occur; e.g. training, practice, or reading can help one be more effective at achieving their values. (See also.)
(My comment wasn’t in tension with that idea; I just didn’t bring that up as I didn’t see that as part of the point you were making in the paragraph I quoted.)
But I think “value drift” away from EA is probably relatively rarely that someone has the exact same values (including “surface-level values”) but “forgets” how to act on them, or develops mistaken beliefs about how to act on them. I think it’s more often either that their “fundamental” values shift in a substantial way, or something like they just stop caring as much about EA cause areas or approaches. The latter doesn’t require that they no longer think what EAs are doing is valuable; they might just feel less engaged or motivated by it. That still seems to me like something we can call “value drift”.
We could also perhaps call it “motivation drift”, “revealed preference drift”, or something else like that. But “value drift” seems adequate to me, for those cases.
I agree that they can be described as effectively pursuing another goal (if indeed they’re doing that effectively). But I don’t think that prevents us from saying they’re “less effective”, as a shorthand for “they’re less effectively achieving good in the world”. And this in turn can be from our own perspective. As I mentioned in another comment, I think people speak in that sort of way very often (e.g., when saying “positive impact”), and that it’s easy enough to understand that that’s what people mean.