As best I could make it out from looking at dates on drawings and talking to the ILM guys that participated in Lorne's booksigning here in LA, the RBR/Falcon was one of the first models started back in the late summer/fall of 1975. It had a fairly complicated base structure with all the boxes and engine tubes that had to be worked out. They seemed to concentrate on that one a lot due to the roll and number of shots it would be in and by the end of the year, paint was being applied and all that. Photos were sent to England to act as a reference for the full size set, including one that had a cutout figure for scale reference. Like the other photo here shows, it has the familiar Falcon cockpit as well as the radar dish. That version of the model also had landing gear. The speculation is that during the review in England, someone made the Space:1999 comment and that blew the whole thing for George. In late '75/early '76, sets were being constructed even though shooting would begin in Tunisia. I suspect that when the decision as made to change the design, someone brought up the fact that construction had already begun on the full size falcon cockpit set (the one that had all the instumentation and the one that all the actors would be in). Since it would be impossible to redsign and build it over again on SW's schedule and budget, I'll bet the mandate came down that the redesigned Falcon must use the same cockpit as the original. (As for why the radar dish made the transition, I don't know). That way construction, wiring , etc could continue on the full size set without delay.
Shots of the saucer Falcon miniature under construction show an open ended tube jutting out from the fuselage - likely they just stuck the finished cockpit piece from the RBR/Falcon on it when it was done and hooked the lights up to the wiring harness. Normally (if there was anything normal about the making of STAR WARS) ships would be built by different groups of people. Grant McCune would work out the general layout and structure. Jamie Shourt would work out the moutnting point and armature. Then, one or two of the guys would detail it out. Dave Jones and Joe Johnston would usually do the painting. All of this was done to create some kind of assembly line and still maintain a cohesive design aesthetic. Well, the Falcon was such a rush job that ALL the detail guys had to work on the model. There was a concern that with so many guys working different sections, that they wouldn't match up aerthetically, but Joe Johnston managed to keep everthing heading in the right direction.
For a time, there was the idea that the ship would rotate into a different position after takeoff while the cockpit would always remain upright (at the top or bottom, like the B-wing from 'Jedi'). Given the size of the miniature (and the weight), there was just no way that was practical. I don't know how far along that idea went before it got shot down. Lorne says that the section immediately behind the cockpit was there to imply that the cockpit could rotate. To my knowledge, the cockpit has no motors in it, so I don't know if they actually proceeded with the idea or if it was just a "wouldn't it be cool if...."
The RBR got a new nose, with some revisions to the windows along the way, and they modified the size of the escape pods. It also got a brand new paint job. The original paintjob didn't have the red stripes and the hull plating resembled the patchwork paint of the saucer Falcon. In changing the RBR over, the scale of the ship really changed. Originally, the main tube running the length of the ship was in scale to the height of a human being - around 6'. Well, with the smaller windows on the nose, they were able to increase the implied length of the ship. Could also be why a lot of the painted panels changed - smaller panels = bigger ship.
As for the McQuarrie art changing - I'll be that was a marketing thing done after the fact. Lucas had two sets of storyboard artists working on the show. The early boards by Alex Tavoularis show the RBR/Falcon and the later ones done by ILM show the saucer. Likely done at two separate times during the span of the show as ILM came to need specific boards to plot out the moves on the motion control stage.
The caveat here is that some of this is an educated guess. I've talked to Dave Jones and Lorne Peterson about this, but there are things that they just don't remember. Dave was part time till later in '76. Lorne didn't come on to the show till December of '75 with (Jon Erland) and he recalls that the RBR/Falcon was almost done at that time, so he didn't work on it. (He did work on the saucer Falcon).
So there you go.....hope this helped or at least entertained.
Gene