For the 3-month cutoff projected shifts fans: required reading
Gastrodon could move from RU to NU
Marowak-Alola could move from RU to NU
Araquanid could move from NU to PU
Guzzlord could move from NU to PU
Inteleon could move from NU to PU
Frosmoth could move from PU to ZU
Ninjask could move from PU to ZU
Ninjask to ZU?
I initally said that Ninjask would be perfectly fine in ZU, but after discussing with other ZU higher-ups, I feel like it could definitely be a centralizing threat. While it has plenty of defensive checks, there's very little (if anything) that can out-offense it. In higher tiers, offensive breakers have the bulk to take a hit from Ninjask and retaliate, or at least take minimum damage from U-Turn. The lower power level in ZU makes the prospect of this much more difficult (with the exception of niche mons like Rotom-Fan). So, I think ZU might be overwhelmed by it.
Frosmoth to ZU?
Before it rose to PU, Frosmoth was a solid sweeper that was far from broken. Not only do I think Frosmoth will be balanced in ZU, but it debatably could be worse than before. This was already made clear by the rise of special walls like Articuno, Miltank, and Coalossal, but the omnipresence of Klinklang makes it even more difficult for Frosmoth. Still though, it's nothing to scoff at, so it would be a decent addition to the tier.