Gengar: A -> A-: Disagree
I am wholeheartedly against this. I am being 100% serious when I say that seeing a Gengar scares me. Even if I have a pursuit trapper like Bisharp or ScarfTar, I don't ever look foreward to having to deal with it; this is because it is just completely impossible to reliably switch in on, and I am always hesitant to send out my Pursuit trapper for fear of having my world rocked by Focus Blast or Will-O-Wisp or Substitute. Sure, the abundance of dark-types is problematic for it, but enough to push it down? I don't think so, as it is capable of beating most Dark-types if it gets a sub up (although Hyperspace Hole/Fury beat out sub, which is annoying), it deters its counters switches with Wisp and Focus Miss, and it has huge utility in almost every match it is brought to. I have only ever come across it not taking a major toll on the match once, and its ability to escape from Gothitelle and Wobbuffet are making it significantly more valuable support for offensive and balanced teams to have. Sure, it is frail, and a single misprediction v.s. a Pursuit trapper can send it back to whichever Clefable's grave it came from, but it has a good speed tier, is so hard for anything that's not blob-shaped to switch in on, its mediocre bulk is easily accounted for by its 3 immunities and good set of resistances (allowing it to easily switch in on a lot of 'mons - which is certainly more than you can say for most other frail attackers like Alakazam or Weavile) and it is heavily splashable. Finally, it is definitely more consistently great than Politoed or MegaCham etc. are.
Mega Slowbro: A -> A-
Not used/seen it enough to comment
Togekiss: B+ -> A-
Same as M-Bro
Celebi: A- -> B+: Neutral
I'm completely neutral on Celebi, as I can see the arguments for both keeping it up and pushing it down. I like the utility it has for a number of teams (Perish Song Celebi is so fun to use, and it has a huge range of other supportive options to choose from), but metagame trends aren't all that kind to it.
Volcarona: A- -> B+: Disagree
This is such a threatening sweeper, and it is one of the easier ones to reach a point at which it can sweep. Whether it is bulky, Lum, Passho or LO, it can very easily sweep the opposing team with only moderate support (a hazard clearer, something to deal with what its choice of coverage doesn't and something to beat Talonflame is all that is needed really (half of these roles are done by Wobbuffet or Dugtrio, which helps)) and it has one of the most threatening typings in the game at this time. Additionally, its ability to beat out sub (most notably from Alakazam) with Bug Buzz means that they can't just sub on everything they force out - which is big considering how dumb they can be to combat sometimes, and its typing means it has an easy time sweeping v.s. a big proportion of teams. You can't sleep on it, and Quiver Dance makes it more threatening than the majority of sweepers, and it is easier than ever to deal with hazards - giving it an easier time to come in than ever before. Finally, the prevelance of darks and the offensive nature of the metagame leave it in a position where it has a very good matchup v.s. a large number of teams. STAB Bug Buzz and Fire Blast in conjunction with Quiver Dance let it rip through such a large portion of the metagame that it is just insane. Of course, I respect it is far from flawless (it struggles if hazards do get onto the field, for example), but I think that the current meta is kind enough to Volc for it to not drop.
Mega-Gallade: B+ -> B: Agree
Mega Gallade is ass. While it certainly has a nice speed tier, it just faces so much competition from other fightings (some of which aren't complete dead weight v.s. Sableye due to things like STAB Stone Edge). I personally think that it should be B- (I'd definitely put it on the level of Lucario, who I'd argue is fractionally better in the meta due to ExtremeSpeed, hitting M-Sab hard with Iron Tail and not taking up a mega slot), but I can understand why the ranking team is hesitant to do so.
Mega-Swampert: B+ -> B-: Agree
Wholehartedly agree. Not having Swift Swim on turn 1 is completely detrimental to this thing, and there are much better choices for rain mega than the thing which has its job done well enough by Kabutops anyway. Why would you use this thing over Mega Scizor (who gives a slow u-turn, reduces pressure on Swift Swimmers with SD, lures Hippowdon and MegaZard-Y so as to allow for instant re-setting of the weather and "loses" its sole 4x weakness), Mega Pinsir (who takes on a large number of big threats to rain such as Ferrothorn, Celebi and Gastro while also reducing the pressure on Politoed to keep the rain up with Quick Attack) or something else (Heracross is of note for its ability to do Pinsir's job against different archetypes, and so is Man 'cause Thunder+Volt Switch is godly)? There are simply better megas to use, and its not like any rain team actually misses having this thing considering that there are other, better swift swimmers to choose from. It just faces too much competition for a teamslot tbch.