Hey, welcome to the second part of my Altering FNaF series! In this part, I'll be going over Five Nights at Freddy's 2.
If you're confused about how everything works, read the beginning of the first part to understand the categories.
Anyway, without further ado, let's get into it.

1) UI

The UI would also change drastically, just like FNaF 1.

Here is the main menu! Just like Five Nights at Freddy's 1, there is now an extras menu and a 4th star. The only other change this menu would have is that the withereds would appear more and more on the menu as you progress through the game, until eventually replacing the toys when you beat night 5.

The new night screens are the same as FNaF 1. Each screen shows one of the new characters that activates on the night you are playing, with the one for night 1 being Toy Bonnie.
That is about it for the UI! Let's move onto the gameplay, one of the most divisive parts of FNaF 2.

2) Gameplay

Just like FNaF 1, I'll talk about the characters before talking about any other gameplay mechanics.

TOY BONNIE: Activates on night 1. If you see him in the office's right vent, put on the mask and wait for him to get out of the office before taking it off. His blackout animation is a lot faster than in the original game.

TOY CHICA: Activates on night 1. If she is in the office's left vent, put on the mask and take it off only after she gets out of the office.

MARIONETTE: Activates on night 1. Wind the music box in the prize corner to keep The Marionette in the box.

TOY FREDDY: Activates on night 2. Watch him on the cameras and do not let him get to the main hallway. He will go from the stage, to the dining area, to the prize corner, to Kid's Cove, back to the dining area, into the bathroom hallway and into the main hallway. Watching him for about 2 seconds will make him go to the previous camera.

THE MANGLE: Activates on night 2. You will always have a radio on the desk in the office. Whenever you hear Mangle's radio, turn on the radio on the desk to make Mangle be unable to hear any sounds you make. You can turn it off after her radio stops. Be careful, as playing the radio for too long will make the other animatronics slowly become faster and faster. If the radio is played for about 30 seconds, the animatronics will get so fast to the point where you pretty much can't fend them off. Turning off the radio slowly calms them down.

BB: Activates on night 2. If you hear him say "Hello!" instead of his laugh or a "Hi!", then search for him. If you spot him in any of the vents or in the main hallway, flash him with the flashlight in order to make him go away. If you don't, he will steal your batteries and never give them back.

WITHERED FOXY: Activates on night 2. If he is in the hallway, hold the light on him until he leaves. Due to his simple mechanic, he will attack a lot.

WITHERED FREDDY: Activates on night 3. If he laughs, then that means his attack sequence has started. Search for him on the cameras and hold the light on him until the ambience sound that plays during this sequence fades out and then stops. Every time he switches cameras, his kill timer resets. Even if the other animatronics don't pause during this sequence, you can still do it. He gives you a lot of time to search for him.

WITHERED BONNIE & WITHERED CHICA: These two activate on night 3. They work as a team. If you hear Withered Chica trying to crawl through the vents, hold the light on her until she goes away. After you do that, you will hear Withered Bonnie's footsteps. Put on the mask before Withered Bonnie enters the office and wait for him to walk out of the office.

WITHERED GOLDEN FREDDY: Can activate on night 4, however, he has a higher chance of showing up on nights 5 and 6. If he is in your office, look at his eyes. If he has pupils, pull the cams up. If he has no pupils, put the mask on. He laughs every time he enters the office.

And those are all of the characters! The only other new mechanic is the batteries, which are like the OG FNaF 2 but slightly different. If you happen to run out of batteries, press the spacebar key to go to a room behind you. In this room, you have only 1 extra battery, so use it whenever you run out of battery. This room can also be used as a last-ditch effort if you're about to die to one of the characters. There is a chance that they won't know you're hiding and they'll leave, but there is also a chance that they will know you're hiding and they wil kill you. The Marionette, Withered Freddy, Mangle, BB and Withered Foxy will always know you're in here.

3) Lore

Most lore would remain mostly the same, however, after you beat the max mode, there would be an 8-bit cutscene of the bite of 87'. Mangle would have done the bite and the victim would have been an employee.

Other than that, Shadow Bonnie and Shadow Freddy would NOT be agony creatures. They'd be the spirits of previous employees who got springlocked but didn't have a suit to possess at Fredbear's.

The blue Paper Pal wouldn't move at all.

4) Misc. Changes

First of all, the obvious one, Extras! (Thank you to FreddyFazbear255 for the W. Bonnie render)
Nothing is super different from the FNaF 1 extras.

FNaF 2 already had Custom Night challenges, so I don't need to mention those.

Now, time for the death voicelines, just like FNaF 1!

Toy Freddy's Voicelines:
-Seems like the facial recognition messed up.
-The mask won't fool me.

Toy Bonnie's Voicelines:
-You're a rockstar!
-There you are!
-This is my chance to shine.

Toy Chica's Voicelines:
-Where's my beak?
-Not laughing anymore, eh?
-I'm sorry... Am I here early?

Mangle's Voicelines:
-[*Radio*]

Marionette's Voicelines:
-I'm not afraid of you.
-I am very aware.
-The mask won't fool me either.

Withered Freddy's Voicelines:
-I have something special, j-just for y-you.
-That b-bright light won't s-stop me anymore.
-Round and round w-we go.
-I hope you e-enjoyed my visit.

Withered Bonnie's Voicelines:
-Let's face the f-facts, you were a-always destined to f-fail.
-Time to f-face the consequences.
-Wait, w-where's my f-face?
-F-face the camera.

Withered Chica's Voicelines:
-I never t-thought I'd make it throught that v-vent.
-I was the first.
-I have s-seen everything!

Withered Foxy's Voicelines:
-Follow my v-voice.
-Listen to m-my voice.
-Be sure y-your step.

Golden Freddy's Voicelines:
-[*Laugh*]

Upon getting all of the stars and clicking on the third one, the game would crash.

Well, that is pretty much it! Huge thanks to @baharkolahi for BB's mechanic and inspiration for the Withered team, and another thank you to @bixqa for Mangle's mechanic and also giving inspiration for Toy Freddy and Withered Freddy's mechanics.
One last thank you to my server members for also suggesting some smaller but still important changes!
Well, that is about it! Thank you for reading and stay tuned for FNaF 3!