I'm not really sure where I stand on Charizard X being A+ or S, however, if it does move down I think a few other things should move down as well.
If Charizard X moves down to A+ here's what else needs to move down as well...
1. Keldeo needs to go back to A+ as well
I'm just gonna quote myself here since ive already talked about this and dont want to repeat the same things over and over
Keldeo isn't as good as other S rank pokemon and lacks versatility. It is over reliant on choice items, which in tandem with it's lack of versatility make it very predictable.
While Keldeo is a great pokemon, it is undoubtedly the worst S rank mon. In comparison, it isnt nearly as versatile as the other S rank pokemon. It's two most common sets are both choice sets with little variety between the moves. The other (less common) sets it runs are expert belt and calm mind. These sets arent nearly as good as the choice sets and dont really get past any checks and counters to the more choice sets as well, unlike other S rank pokemon, such as Mega Mawile. Mega Mawile has only two sets yet it is far more versatile than Keldeo as both sets have a different list of checks and counters, while Keldeo's sets all struggle with the same set of checks and counters. Of course, Keldeo can change its coverage move to adapt to hitting these checks and counters, but its not that easy. All of Keldeo's coverage moves are piss weak and cant reliably 2hko these checks and counters without choice specs. With choice specs keldeo has to predict the switch in correctly and if it is wrong it is locked into a low base power move. Even if it is right the checks and counters could switch out, leaving Keldeo locked into its low base power coverage move. Keldeo has this problem on all of its sets as well. As good as it's choice sets are, being locked into one single move isnt ever a good thing. It is forced to predict correctly or lose a ton of momentum. Lets say Keldeo KO's something with a water move. Now Ferrothorn can come in and set up stealth rock or terrorize switch ins with Leech Seed + Protect. Or Ferrothorn could predict correctly and switch in, or it could switch in after Keldeo damages something with a water move. Obviously Ferrothorn isnt all that can take advantage of a Keldeo locked into a move either, it was just my example. The Mega wall breakers can all come in on secret sword and wreck shit too as well as anything that resists the move Keldeo is locked into. However, the most important reason why Keldeo should drop is that the metagame is slowly shifting away from it. With the banning of the Deos and now Aegislash the metagame has shifted drastically. Bisharp, whom Keldeo was offenses' best answer to, has gone down in usage. With the lack of Aegislash and hazards birdspam has gotten a lot better too. Pinsir is now free to run Close Combat and Staraptor is that much better at wall breaking, both of which greatly bolster birdspam's popularity as a playstyle. After the banning of Aegislash the Lati@s have improved a lot and they are both some of the best switch ins to Keldeo. Starmie is a lot more viable too and the bulky set with recover, reflect type, and natural cure is gaining in popularity which makes Keldeo's life that much worse. Assault Vest Azumarill, one of the best glues to offensive teams, is now better and more common than ever as it is no longer cock blocked by Kings Shield. And the icing on top of the cake (although it doesnt apply currently) is that Mega Mawile is going to be banned. Keldeo is one of offenses best checks to that monster because it resists Sucker Punch and can outspeed and OHKO with specs Hydro Pump (I think). Due to this combined with Keldeo's lack of versatility and dependency on choice items are why it should go back to A+ and why it probably shouldnt have gone to S to begin with.
Now that Mega Mawile has been banned the Lati@s are now even better and more common than ever, which is bad news for Keldeo as they are some of Keldeo's best checks/counters. Starmie has gotten even better too and the bulkier sets can reliably check Keldeo throughout the match. Assault Vest Azumarill's been increasing in popularity as of recently too and is another great answer to Keldeo. Finally, Mega Mawile is gone so Keldeo lost its niche as checking one of the most dangerous forces in the meta for offensive teams as it is one of the few things that resists sucker punch.
In short, I think Keldeo should go to A+ regardless, but under no conditions should Mega Charizard X go to A+ and Keldeo stay in S. Mega Charizard X is simply better and more versatile than Keldeo (I'm sure others can back me up on this if needed), and shouldnt be a rank lower than Keldeo.
2. Mega Tyranitar should drop to A
Too be honest Ive never really been a fan of Mega Tyranitar and wanted it to drop for awhile, however basically nobody else agreed with me until now. It has one viable set and many common checks and counters that are on the rise such as Sand Rush Excadrill, Scarf Landorus-T, and Ferrothorn. It faces stiff competition from Mega Gyarados and Mega Charizard X, both of whom are also bulky Dragon Dancers that take up a mega slot as well. Being widely considered the least effective of the three (Ill elaborate on this if needed but people generally agree so i dont want to waste my time) it shouldnt be in the same rank as the other two, especially Mega Charizard X. Therefore, Mega Tyranitar should drop to A.
3. Landorus could drop to A+
Ive never really had a problem with Landorus, ever actually (still mad about that gen v ban :p). I understand that it apparently destroys stall, however, it has plenty of good checks and counters that stall teams can use (Sp. Def Gliscor, Cresselia, Mew, AV Tornadus-T, Lati@s, Charizard Y, Zapdos, Sp. def Celebi, AV Slowbro (ive seen it on stall before), Sp.Def Rotom-W, or even stuff like Garchomp if they have too because it can take a hit and outspeed Landorus. Of course I'm not arguing Landorus's effectiveness against stall, I'm just saying its not some sort of god that 6-0s stall and makes stall unviable like people make it out to be. But the main reason I want Landorus to drop is because its not nearly as effective against the tiers most common playstyle, offense. Obviously it isnt as bad as Mega Heracross, Mega Medicham, and Mega Gardevoire are against offense, but it isnt amazing either. 101 speed isnt fantastic anymore and is outspeed by many member of offense. Landorus doesnt exactly have the greatest bulk so it really cant take a hit from many of the things that outspeed it either and due to this it often loses momentum for it's team. Finally, using Landorus also means you cant use Landorus-T, who checks a large amount of things for offensive teams and has an amazing scarf set. Despite all of this I could still see Landorus in S, as long as Charizard X stayed there too, however i wouldnt mind moving it down to A+ anyways.
4. Clefable could drop to A
As long as Charizard X stays in S, I wouldnt mind seeing Clefable in A+. Unfortunately though, being one of the weaker members of A+, I think it would have to move down if Charizard X did too. Despite having a good typing, abilities, and many viable sets, Clefable's raw stats often leave much to be desired and many top tier threats can usually muscle through it even when attacking whatever defense stat Clefable invested in. As good and annoying as Clefable may be, I just cannot see it in the same rank as Charizard X.
5. The Mega wall breakers should remain in A
Regardless of where Charizard X goes these guys should stay in A. I am really tired of typing so Im going to make this quick, they are simply too ineffective against offense and also face competition from other stall breakers such as Mew and Landorus who dont take up a mega slot. Putting them in the same rank (if Charizard X drops to A+) would be wrong and terribly inaccurate.
I'm not saying all of these should happen, only if Charizard X drops down to A+ should some occurr. If Charizard X stays in S, I'd like Clefable to remain in A+ and Id like Landorus to drop to A+ anyways, although I doubt many will agree here so I guess it can stay in S. However, the Mega wall breakers should stay in A regardless, Mega Tyranitar should go to A, and Keldeo should drop to A+. But, if Charizard X drops to A+, all of the changes I mentioned should happen, but maybe with the exception of Clefable and I suppose I could live with Landorus being a rank higher than Charizard X if I absoltuely had to.