Elvis brought Rock and Roll to millions. He wasn't the first and didn't write his own stuff - but that's not what Pop music is about to this day.
The Beatles weren't the first to play Beat music, or use any of the techniques that they did - but they raised the bar several fold and were/are extraordinarily influential.
However, in terms of bringing something entirely new to the world of music and changing it forever, they didn't and couldn't do as much as Elvis;
Elvis took charisma to a new level - his talent lay in "being the King of Rock'n'Roll", not in any particular musical skill, but the sound his band and voice made combined with the huge publicity machine that created itself around him - not least because of the way he moved and made the girls scream - ensured that no-one would ever have the same impact on the world of music.
Style over substance - that's the major change that Elvis brought. And that's what pop "stars" do to this day.
The Beatles (thanks to George Martin) combined the two, but Elvis had already blazed the trail for them.
So Elvis changed it more, IMO.