Derek Ueckermann was just the guy who took the photographs. Not exactly sure what he's doing on this but he works in the camera department. The DoP for RBW Block 2 is actually Ross Emery (Scott's second unit DoP on Alien: Covenant). I'm assuming that Wolski worked with Scott on Block 1 but I can't actually confirm it as yet. Scott would probably want Wolski on Merlin a little later this year, so I guess he didn't want him tied-up with this very lengthy production.
Changing cinematographers during production on feature films certainly isn't common practice, and when it happens it's usually not a good thing. Happened on Aliens and Alien 3. But in this case it's probably different since there is also a change of director (and I think several of the prime crew members).
