According to Businessweek's Lucas apparent even begin formal discussions with stars Mark Hamill, Harrison Ford and Carrie Fisher about returning to the series that made them famous. He also apparently had arranged for for the treatments by bringing in screenwriter Michael Arndt and consultant and Star Wars scribe Lawrence Kasdan before even agreeing to show the Disney company the rough outlines for the new trilogy.
In terms of casting (i.e. Luke, Leia and Han) Lucas was quoted as saying:
"We had already signed Mark and Carrie and Harrison—or we were pretty much in final stages of negotiation. So I called them to say, ‘Look, this is what’s going on.’ '
'Maybe I’m not supposed to say that. I think they want to announce that with some big whoop-de-do, but we were negotiating with them.'
So that pretty much confirms there’s an absolute intention to have the three characters in the first film at least (but these days contracts for franchises extended to multiple films) – with Carrie Fisher saying she’s up for it and Mark Hamill practically begging to be in it, the only real thing to worry about is if grumpy old Harrison Ford will sign up.