EW has some details after interviewing Ron Howard and Kathleen Kennedy about the transition, and how Howard felt when he came on board.
Howard speaks about his relationship with Lord and Miller:
“It’s rough on everybody and disappointing for everybody, and I’ve just tried to come in and — of course, Phil and Chris’s fingerprints are all over the movie, given how much they put into it and the time they put into it. I hope fans won’t even think about how the movie was made. They should just lose themselves in it.”
He talks about his time on the set of American Graffiti with George Lucas:
“I said, well, do you know what you’d want to do after American Graffiti? And he said, ‘Yeah, I want to do a big science fiction movie. I’m working on a script.’ And I said, “Well, what is it?’ And he said, ‘Oh, it’s a little bit like Flash Gordon, but it’s not Flash Gordon, but I liked those movies when I was a kid and those comics and things.’”
One day during filming, George Lucas stopped by the set, just for a few minutes to say hi. He was there for 5 hours, and apparently put on his director's hat for a moment, according to Kennedy:
“He had intended to just kind of stop by and say hi, and he stayed five hours,” Kennedy says. “There’s even one little moment in a scene that — I can’t tell you what, sorry — but in the scene on the Millennium Falcon where George said, ‘Why doesn’t Han just do this.’ It actually is a funny little bit that will probably get a laugh,” Kennedy says. “And Ron happened to be by the monitor and not inside the Falcon and he goes, ‘Oh that’s a great idea,’ and ran in and said, ‘George wants us to do this.’ So that was pretty cool. I think George felt pretty great about that. He could revisit these characters, and I think he felt so comfortable, obviously with Ron being there, that it was just fun for him."
“What [Ford] did so beautifully for Alden was he talked a lot about what he remembered when he first read Star Wars, and what George had done with Han. Who the character was and the conversations he had for so many years with George about how that character developed. He gave Alden that kind of insight which was invaluable. There were several times in the course of making the movie where Alden would actually recount some of the things that Harrison had pointed out. I think that was really, really helpful to him.”