SPOILER ALERT! YOU HAVE BEEN WARNED!
CBR's Rich Johnston is saying that preview's climactic ending was filmed two ways: The true to comic book ending complete with a giant alien squid transporter accident; and the Dr. Manhattan inspired nuclear explosion, and that the latter version may indeed have been a test by the moviemakers to see whether or not they could get away with deviating from the comic book.
Here's the quote:
"I'm told that this screening was intentionally leaked to the fan press - with the exact time and place posted online with details of how to evade the security. The intent is to gauge fan reaction to a squidless ending for Watchmen and see what they can get away with, believing it to be more suitable for a more mainstream audience. The FX for the squid has been completed however."
If this is true, then pop-culture website Collider was in on it, since they originally leaked the info and instructions...
AND, if Zack Snyder or anybody at Warner Bros. wants the final word on changing the ending straight from hardcore comic book movie fans. Just check out our poll HERE.