Article on BTTF.com
Another article about the specs
Another thread about the 4-disc
Basically from what I can gather it's trying to fix the framing mistake in the first set. Also, giving the people outside of the R1 users to get the bonus features that were only availible previously to the R1 set. So, basically if you have the R1 corrected set already you don't need to get that set. If you want the R1 corrected set you can buy it here. Also, here's a webpage with more information about it. on that page you'll also, find numbers/e-mails to (mabye) get the V2[corrected] disc's. I don't know if there still sending them out because, it's been a long time since. Also, on that page there's images to show how to determine if you have the corrected set.
here's also, another F.A.Q. about it
Here's another thread with addresses
Keep in mind the above threads are old...
Also, if you want to know why it happened in the first place. I'll tell you. The company they hired to do the transfer mistakenly used the ILM 1:66 plates for the transfer of the II and, III DVD's. Thus, the incorrect framing. So, Robert Zemekis or, Bob gale(I forgot which) found out about it and, got Universal to fix the problems and, offer replacements. Universal didn't advertise about it though, so those that got the fixed versions are basically the one's that were lucky enough to find out about it. Thus, the V2 versions are somewhat of a rarity.
Plus, the V2's were also, re-released but, not advertised as, the fixed versions. Just "the lowest price ever" bttf.com article about it. The only way to distinguish the V1's from the V2's on store shelf's is the orange sticker saying "All 3 Movies! Lowest Price Ever". Although, I don't know if they are still on store shelf's.
Additionally, the only reason the four disc release is happening at all is becuase, of the mis-framing. If that never would have happened, then there would only be one version out of BTTF trilogy per region(and, some stray single BTTF 1 DVD's out there). Yes, the R1 version would be the version with more extra's but, that's because the other regions were released before the R1. Thus, didn't get as, much extra's.
Edit: Oh, yeah, and, I almost forgot to mention. I read that some people were conteplating on a BTTF 4. Well, either Robert Zemekis or, Bob gale own the characters and, they don't want to do a BTTF 4(Universal owns the name Back to the future). So, if there is a BTTF 4 it won't be with Marty, Emmit, Clara, George, Biff, basically anyone in the first three.
