Another place this problem has come up for me is in wanting to ensure representation sharing for values constructed by data constructors which don't make use of their type parameters. A trivial example would be sharing the representation of Nothing between all the Maybe types, or sharing the empty list among all the list types. That example isn't especially motivating, but there are other cases where we can end up with a large structure for which the type parameters are 'phantom' even though they may not be phantom in general (because other data constructors make use of them). That we want type updates for records with phantom types is just a symptom of the larger issue of wanting type updates for all quasiphantom/pseudophantom types.
I feel like this is similar (or the same) as an issue I've had like: data X a = A a | B Z Z | C Z | D Z Z Z -- | Cast the type by stripping out an A. cast :: X a -> Maybe (X b) cast x = case x of A _ -> Nothing no_a -> Just no_a Except of course that no_a isn't going to work, I have to write case branches for B, C, and D and reconstruct them exactly the same way only with a different type, even though it's a type they don't depend on. I assume sharing is destroyed, but it seems like it should be possible to keep it, just like the various types of Nothing and [].