Trainers, it seems that some unannounced Team GO Rocket lineup changes have happened, affecting the GO Rocket Grunts and Rocket Leader Sierra. Niantic has been silent about these changes, but we assume they were made in an effort to make certain Grunts more balanced, and to make the new Shadow Rhyorn more accessible.
As of the time of this writing, we have confirmed and verified with our Japanese colleagues that the following changes have happened:
Team GO Rocket Grunts

- Shadow Rhyhorn is now available in the first slot of Ground-type Grunt battles (“You’ll be defeated into the ground!”)
- Shadow Magnemite is back in the first slot of “Get ready to be shocked!” – Electric-type Grunt
- Shadow Alolan Muk replaced Shadow Muk in the second slot of “Wherever there is light, there is also shadow.” – Dark-type Grunt
- Shadow Wooper, Shadow Blitzle and Shadow Shinx have been removed from their respective Grunts
Team GO Rocket Leader Sierra

- GO Rocket Leader Sierra no longer uses Shadow Kingdra as a third slot Pokémon, it was replaced by Shadow Houndoom
Our thoughts
The addition of Shadow Rhyhorn as a potential first slot Pokémon for the Ground-type Grunt is a welcome change, as it makes Shadow Rhyperior much easier to obtain. Initial data suggested that Shadow Rhyhorn was previously catchable in 12% encounters following a fight with the Ground-type Grunt, which made it very, very difficult to get.
The removal of Muk in favour of Alolan Muk is also good, as it makes it safer to use a Fighting-type based lineup against the Dark-type Grunt. You should probably consider using Ground types in addition to fighters, but still, one less headache to worry about.
Sierra should be significantly easier now that Houndoom is a potential enemy, as Kingdra was arguably a more difficult opponent than Houndoom.