I think I've amassed a list of all the characters that should be upgraded to 2-A, along with reasoning for their update.
At least 2-C -> 2-A
Abraxas - Scales above Well Fed Galactus and killed all versions of Galactus throughout the multiverse
Post-Retcon Beyonder (First Key) - Has half the power of a Cosmic Cube
Post-Retcon Beyonder (Second Key) - Stranger than his first key, has the full power of a Cosmic Cube
Celestials - Equal to the Mad Celestials
Death (First Key) - More powerful than the strongest Celestials, should scale above Well Fed Galactus and should scale above Cosmic Cubes
Edifice Rex - Feared by Lord Chaos, Master Order, The In-Betweener, Infinity and Galactus
Eternity (First Key) - Superior to the strongest Celestials, Cosmic Cubes and Well Fed Galactus
Galactus (Third Key) - Was going to destroy all universes in his clash with Scrier and The Other
The In-Betweener (First Key) - Stalemated Well Fed Galactus Infinity (First Key) - Equals Eternity
Lord Chaos (First Key) - Stronger than The Celestials, should scale to the likes of Eternity, and scale above Well Fed Galactus
Kobik - Is a Cosmic Cube
Mad Celestials - Would have eventually defeated Adult Franklin Richards
Master Order (First Key) - Equals Lord Chaos
Molecule Man (Second Key) - Equal to Cosmic Cube Beyonder and caused a trans-multiversal clash with him
Phoenix Force (First Key) - Should scale to other universal abstracts
Protege (Second Key) - Had the power of Post-Retcon Beyonder
The Stranger - Occassionally portrayed as having power relative to the abstracts (Should probably be changed to "At least 2-C, likely 2-A")
Thanos (Second Key) - Has full control over a Cosmic Cube
Thanos (Third Key) - Defeated several universal abstracts and Celestials, the Soul Stone alone contains a multiverse inside it
Dormammu - Should scale above Classic Doctor Strange's artifacts and spells, scales above Nightmare, is scaled to Eternity, defeated Slorioth, who threatens Eternity by simply existing
SHuman-Gorath (Second Key) - Scales above Dormammu and Slorioth
Zom (First Key) - Scales above Classic Doctor Strange and Dormammu
Doctor Doom (Third Key) - Has the power of Galactus and a Cosmic Cube
Franklin Richards (First Key) - The Celestials consider him a threat
Franklin Richards (Second Key) - Stronger than his first key, scales above Well Fed Galactus, killed a Mad Celestial
Marquis of Death - Could control several abstracts at once
Apocalypse (Second Key) - Stated to rival a Celestial
Jean Grey (Third Key) - Has the full power of the universal Phoenix Force
Cosmic Cubes - Kubik and his equal (Super-Adaptoid) could have destroyed infinite dimensions and adjacent universes by fighting, used to defeat Doctor Doom with Galactus' power, a Cube was used to imprison several Celestials, five Cubes could keep Eternity catatonic
The Infinity Gauntlet - Same as Thanos' third key
2-C -> 2-A
Uatu - The Watchers are scaled to The Celestials (Should be changed to 2-C, possibly 2-A)
Classic Doctor Strange - His 2-C tiering is scaled to Lord Chaos, Master Order, The In-Betweener, Eternity, SHuman-Gorath and The Infinity Gauntlet)
Questionable 2-C -> 2-A
Nightmare - Put Eternity to sleep by channeling 1/3 of the world's dreams
Feel free to tell me any characters I missed, or if you think there's any characters that maybe shouldn't be here.