In chronological order:
In BW, wild Basculin of both colors mistakenly had Reckless, in B2W2, this was changed so that Blue Striped Basculin's first ability slot was Rock Head. However, a Blue Striped Basculin obtained from an in-game trade in White has Rock Head, making this the only case to my knowledge of a Pokemon being able to have four different abilities in the same game.
From Gen V to Gen VI, some Pokemon's Hidden Abilities were changed:
Zapdos: Lightning Rod --> Static
Venipede line: Quick Feet --> Speed Boost
Litwick line: Shadow Tag --> Infiltrator
From Gen VI to Gen VII, another group of Abilities were changed:
Gengar: Levitate --> Cursed Body
Raikou: Volt Absorb --> Inner Focus
Entei: Flash Fire --> Inner Focus
Suicune: Water Absorb --> Inner Focus
Several Hisuian Pokemon's abilities were changed from PLA to Gen IX, despite abilities not being usable in PLA:
H Typhlosion: Flash Fire --> Frisk
H Samurott: Shell Armor --> Sharpness
H Decidueye: Long Reach --> Scrappy
H Sliggoo line: Overcoat --> Shell Armor
H Sneasel: Poison Touch --> Pickpocket
H Growlithe line: Justified --> Rock Head
H Braviary: Defiant --> Tinted Lens
Kleavor: Steadfast --> Sharpness
Basculegion: Rattled --> Swift Swim
Enamorus: Healer --> Contrary
With the release of The Teal Mask DLC in SV, two more Pokemon have changed their abilities:
Shiftry line: Early Bird --> Wind Rider
Empoleon line: Defiant --> Competitive
An extensive list of Pokemon have gained new abilities from game to game; you can see that list here if you're interested.
Hope I could help!
