I loved this film. I wrote a piece on the footage's preservation for Films in Review when it was discovered. I saw it in the theater, bought it on VHS, laser disc and then DVD. I even have the half hour teaser short the AFI did after the footage was discovered. (It is a lot like the first half of the released movie). I spoke with someone at the UCLA archives shortly before the film came out. I was told the producers and Dick Wilson had a specific list of shots they wanted. Of course, UCLA wished they were taking ALL of the negatives for processing and restoration, but that was not what they had in mind.
As a side note, Paramount (which owned RKO's vaults when this was "found") did NOT donate the color footage to UCLA, just the B/W. The color footage was considered more valuable because it was color (I know, I know). As far as I know it is still at Parmount, though they were kind to allow its usage in the film.
Another tidbit (from the documentary script and not the finished film), when a Paramount exec called Welles shortly before his death and told him of the find, the first thing he said was DID YOU FIND MY AMBERSONS?