It also would not be surprising if Kathleen Kennedy (Lucasfilm president) saw what Abrams was concocting and visions of the prequels flashed before her eyes. So she went to Kasdan who she knows from when they both worked for Steven Spielberg and asked for help.
That would embarrass Abrams big time
Who knows? But I do agree that the timing of Kasdan coming in coupled with the delay in production makes it look like what you suggest.
Disney is far more interested in sure things than not, when it comes to acquisitions. It needs to make a strong hit on this next Star Wars film if they want to continue making money on it. They can't allow it to dead end after 5 or 6 years after a hack like Abrams slings mud on it. I am certain more than a few had reservations about the last Trek film and the rather public anger of the Trek fanbase (despite earnings). Disney keeps it's icons alive for as long as humanly possible. Mickey Mouse is over 80 years old and the tiniest of toddlers know who he is. 
That actually is not farfetched at all.
Kasdan will be given license to throw out stuff that does not belong...hopefully.
Funny, but somehow sad. They put cheese on Star Wars and served it in a Happy Meal.
