I'm rushing a bit with these noms since I wanted to gain more evidence but I have limited time to get these in so here goes anyway:

(Psychic) D -> C
As one of the few dubiously sane people trying to use psychic seriously, I've experimented with a ton of different builds for the type and find myself coming back to this mon more often than I think is normal for a D-rank mon. While it is a passive Pokemon most of the time with little utility to offer, it being a defensive steel with Stealth Rock and also being a consistent check to Excadrill allows it to role compress a ton, allowing psychic to run far more options for either screens or terrain teams. I know there is a ladder user that lies around the 1600s that uses a bronzong psychic team and after using it myself I do think there is a lot of potential this mon can offer.

(Dragon) A -> B
Raging Bolt has taken dragon by storm lately and I often don't see dragonite a ton on dragon as a result, if anything it seems like its a bit rare. I think in the current meta its not as good as it usually is due to stuff like Roaring Moon just being better at immediately answering threats like Flutter Mane and also not needing a turn of setup to do so. Band gets worn down too much by rocks and the boots set doesn't mesh well with most modern dragon teams, so I personally think that as of right now Dragonite should fall down a bit due to it not being of the same caliber as its A-rank peers. Raging Bolt, Roaring Moon, and Latios are all super strong and cover a wide range of threats while Dragonite comparatively comes up short and seems to fit more into the B-rank, it doesn't feel like a near auto-include anymore and feels far more niche than the others.

(Fire) C -> B
I think there being a massive gap between the a rank and c rank mons is genuinely insane after playing around with the type for quite a bit, as there are def a few mons from c rank that are a bit better than the others, so I'll be doing quite a few noms for fire in this post. This being said, Skeledirge might be the most controversial of these noms due to the mon being a general momentum sink and seemingly not fitting well on most of the successful fire teams. I argue that its unique combination of spinblocking and unaware utility in a meta with setup sweepers like Volcarona, Dragonite, and non-toxic Gliscor allows it to fulfill a great niche that allows it to work on teams that are more geared towards bulky offense. I think despite the fact it sinks momentum its utility and defensive profile is too good to pass up, especially if you want a spinblocker and don't want to use the frail Ceruledge for that. It should also be noted that while in the Sun, it can take water hits from Pokémon like Urshifu-R pretty well, and not have to be afraid of Swords Dance boosts.

(Fire) C -> B
I think with the prevalence of Flying makes using Arcanine a very strong option, especially since it greatly threatens defensive Gyarados which its other physical attacking peers tend to struggle with. It's rock weakness is definitely not something to ignore, because it limits the amount of times it can come in immensely, but I think with good usage of Court Change or other hazard removal it can shine really well.

(Fire) C -> B
Even with water not being as prevalent or as problematic due to Sun being as powerful as it is, along with Volc's water STAB being negated by Sun, I still believe Volcanion has a strong defensive profile with its ability to stuff Flip Turn opportunities and turn it into pressure. It also has a good matchup into Primarina, who can be really annoying for Fire teams to deal with. I also argue that it is a great asset in the rise of rain water on the ladder, as it forces barraskewda to pick between using its STAB or resorting to the weaker Close Combat, and that decision could allow fire some more breathing room in that MU.