Does anyone know why Chevrolet changed the intake manifold on the early 1968 427/435 casting # 3919852 to the Late 1968 casting # 3937797. Both aluminum. 

Both are 435 hp Tri power, square port intakes. The 852 intake was used in '68 only. The 797 intake was used for '69, but did make it onto some late '68 cars. Iirc, the 852 was considered a "low rise" intake, whereas the 797 is a "high rise"design. Performance-wise, the 797 is the better intake. Depending on what you're trying to accomplish, either intake will work for your application. If you are trying to get it Judged, then I would suggest using the manifold design the car originally had, going by the build date of the ENGINE. Most folks won't be able to tell which one you have, except seasoned Judges. There is a large visual difference between the two, if you really look.

btw...a March build date is NOT early. '68 assembly started in August of '67.... Not really a "late" build date, either. I would have to assume, for your purposes, that the car originally had the 852 intake.

|UPDATED|9/9/2015 7:36:12 PM (AZT)|/UPDATED|

There is only so much room between the air cleaner and the hood of a 427. I did not realize they had a low intake and a higher intake. Both 68 and 69  427 hoods are the same. Looking at both manifolds I really can not tell any difference. How much thicker or higher could it be on the 797 over the 852. Why was it necessary to go to another intake for the 69. The 1967 427 was a different number from the 1968 and why? 

Goin only from memory here. I'm not sure WHEN the "early/late" designation starts for the '68 cars, as far as the intake. I can dig deeper when I get home for more info...
The only reason GM would have assigned a new casting number for a part would be because there was a change in the casting. It's not necessarily even a difference in material, either, since there are some iron parts that have the same casting # as aluminuminum parts. '68 was the last year for the oil filler tube in the intake, for one. Also, the alternator mounts were changed in  late '68, so the front of the intake was also changed to accommodate that. The 797 manifold was also available as an oval port in '69. So basically, any minor OR major change could call for a new casting, hence a new casting number. It depended on the engineer responsible for the part/change.
If you have both manifolds, could you take some pics of both and post them? Perhaps we can find a specific change that isn't overtly obvious.