This question requires distinguishing current values from idealized values, and values (in charge) of the world from values of a person. Idealized values is an unchanging and general way of judging situations (the world), including choices that take place there. Current values are an aspect of an actual agent (person) involved in current decisions that are more limited in scope and can't accurately judge many things. By idealizing current values, we obtain idealized values that give a way in which the current values should function.
Most changes in current values change their idealization, but some changes that follow the same path as idealization don't, they only improve ability to judge things in the same idealized way. Value drift is a change in current values that changes their idealization. When current values disagree with idealized current values, their development without value drift eventually makes them agree, fixes their error. But value drift can instead change idealized values to better fit current values, calcifying the error.
Values in charge of the world (values of a singleton AI or of an agentic idealization of humanity) in particular direct what happens to people who live there. From the point of view of any idealized values, including idealized values of particular people (who can't significantly affect the world), it's the current values of the world that matter the most, because they determine what actually happens, and idealized values judge what actually happens.
Unless all people have the same idealized values, the values of the world are different from values of individual people, so value drift in values of the world can change what happens both positively and negatively according to idealized values of individual people. On the other hand, values of the world could approve of value drift in individual people (conflict between people, diversity of personal values over time, disruption of reflective equilibrium in people's reasoning), and so could those individual people, since their personal value drift won't disrupt the course of the world, which is what their idealized values judge. Note that idealized personal values approving of value drift doesn't imply that current personal values do. Finally, idealized values of the world disapprove of value drift in values of the world, since that actually would disrupt the course of the world.
Agreed. In the abstract, value drift is just a change over time, within an agent or across a line of replicated agents. And from the point of view of any given set of values, any other set of values is "bad". If the other set were good, the agent would seek to adopt them, right?
So the question really is whether value diversity has meta-value. diversity over time isn't that much different than diversity across agents.