Everything was pretty much the same up to the meeting with the Star Child. In the original versions there was no Star Child (other interviews have said it was added on the order of Casey Hudson). Instead, when Shepard reaches the Citadel master control room he/she essentially jacks a control wire into his Omni-tool and it brings up a virtual reality projection around him where he meets the “Reaper Queen”, who explains to him that the Reaper’s harvesting agenda is not sustainable and will eventually lead to the extinction of all life because the Reapers cannot evolve. When the Queen tried to explain this to the other Reapers they trapped her conscious inside a virtual prison in the Citadel where she has been stuck for a very long time. The Reaper Queen offers to help Shepard end the conflict, and Shepard is then given three endings to choose from.
Renegade Ending: Shepard says fuck you and all the Reapers, goes ham on the Citadel master control area and destroys it, the Citadel explodes, with the resulting shockwave destroying all the Reapers but also killing everything and everyone still on Earth. The rest of the galaxy, however, survives.
Paragon Ending: Shepard says fuck you to the Reaper Queen and erases her, and assumes control to become the Reaper King, losing his body but becoming the new Reaper AI. Shepard takes command of the Reapers and becomes a benevolent AI-god-type thing and ends the war.
Synthesis Ending: Shepard agrees to help the Reapers evolve so they will reach the same conclusion the Queen reached and stop the harvest. Shepard’s body combines with the Reaper Queen AI and they use the Citadel/Crucible to do pretty much what was done in the synthesis ending we got in the retail release.