What was the original ending and why was it changed?

I don't quite get what you meant by,

I found it weird that the first ending would not compromise the other (it's like a happy ending where the viewer doesn't know what happened to his mother or to the main character's "gift")

Anyway, the way I understood is this:

The original ending was the ending which Evan was in a psychiatric hospital- he is in a really depressed state as he is fully aware that it is he who killed his girlfriend, Kayleigh. What the doctor said was true- Evan was making alternate realities in his mind to escape his guilt about killing her (he throwed the dynamite at her father, but it landed beneath her).

But then he steals the earliest home-movie tape in which he had met Kayleigh for the the first time in his life (the real one)- at a party. He then "travels" into the scene (for real) and then blames Kayleigh (says he hates her, etc, etc.), thus avoiding ever meeting her and playing with her- and the dynamite scene and child abuse (by her father) would never have happened in the first place. And consequently, Evan would not be crazy, his other friends would be good in life, and he would have led a normal college life.

This is not based on real facts, but is the way I interpreted the movie and understood it.