I've played through the DLC last night (making sure to disable auto-save before choosing so I could re-load at the decision point) and have watched all the endings I know about. Here's what I remember from my playthrough (I suspect they'll be differences and tweaks depending on what each players had done during their game, who is still alive, etc).
There's so many spoilers in this post and I'm only going to hide a few of them. You've been warned.
There is a brand new ending available, which can be triggered by:
Shepard can fire her weapon at the Catalyst (this is what I did to trigger it) or apparently Shepard can refuse to make a choice during the conversation.
This results in:
Shepard is left standing on the Citadel.
The Reapers implicitly defeat the current races of the galaxy. Everyone presumably dies.
No elements of the original three endings are shown.
You then get to see a time capsule left by Liara, presumably informing a future cycle of organic life about what happened.
There is a new post-credits scene (of the same type the other endings have - an adult and child talking) that seems to imply this information left by Liara has allowed a future group to break the cycle and defeat the reapers.
As for the original three endings, the following happened (with changes bulleted and any differences between the endings noted) for me, other playthroughs may be slightly different:
After Shepard makes her decision:
Admiral Hackett will broadcast a message telling the fleets the crucible is activated and to retreat.
A shot of the Normandy bridge shows Joker resisting, before a squad member (it seems this changes, don't know what determines this) encourages him to go. Joker looks downtrodden and we see a shot of the Normandy retreating and jumping away.
We see other ships of the fleets retreating too; they jump away from the scene.
Then the first blast of energy is released from the Citadel, hitting the reapers in orbit.
Followed by the shot of ground troops fighting in London, when the energy blast hits.
- Next we see Steve Cortez (the shuttle pilot) aiding a lone injured solider in a battlefield. There seem to be no other solders about, and are about to be overrun by husks.
- In one ending (can't remember which) a husk pounces on Cortez, pinning him to the ground, then the energy hits.
- In the others the energy hits before the swarm reaches him.
- In all endings Steve was left alive (for my play through anyway, I suspect there might be a "bad" version if you did't befriend him?).
Next is the sequence showing the Citadel firing an energy beam at the local relay.
The relay still 'overloads' and explodes with energy, but:
- The explosion has been lessened, the implication now being the relay is no longer destroyed, just damaged.
The energy explosions are seen to spread throughout the galaxy via the mass relay network.
- Next are a few shots of the ground troops on other planets witnessing the energy arrive (and either defeat or change the reapers, as per London). I witness Asari and Krogan, but I've seen Turian online as well. Don't know what determines which you see.
Then is the sequence showing the Normandy running from the energy blast.
- The internal shots are much calmer. We see personnel in the main bit on the bridge level (where the galaxy map is) and Joker piloting.
- It seems the Normandy is no longer damaged; there's no fire in the cockpit, etc.
- We no longer see the blast hit the Normandy, but the implication is that this does still happen.
This is followed by the sequence showing the Normandy on an unknown planet.
This seems to play out the same as before except that:
- The Normandy no longer is shown as damaged - no scorch marks, fire, sparks, etc.
Next comes an entirely new sequence.
- We hear a monologue, spoken by (I won't detail what is said as I can't remember details, and all three are fairly long just provide additional insight in to the following scenes):
- Admiral Hackett (Red / Destroy ending)
- Shepard (Blue / Control)
- EDI (Green / Synthesis)
- While this plays we see still shots of characters we have met / helped during the game (such as Miranda, Jacob, Jack, Samara, Wrex and/or the Krogran, and more seem possible), these all seem positive. A few shots may show characters who have been lost (such as Mordin, Thane, probably more).
- All three endings then imply that the races of the galaxy shall work together to rebuild what as been lost, including the mass relays and Citadel.
- In the Control ending the reapers are assisting with the reconstruction efforts.
- Next is a sequence showing the surviving squad mates on the Normandy, adding the name of Commander Shepard to the center of the memorial wall.
- Admiral Anderson's name has already been added to the wall (also centered).
- In the Destroy ending, the names of Legion and EDI are both present on the wall and EDI is not seen with the other squadmates.
- We see the Normandy take off the planet it had landed on and fly away.
The 'Shepard might be alive' scene (showing scorched N7 armour in wreakage and a sudden inhale of breath), is still present at the end of the Destroy ending.
The credits roll.
The final sequence showing an adult and child discussing The Shepard is still present (and I didn't notice a change to this, but might be subtle).