The announcement that the Watchmen movie was, after two decades off and on in the planning stage, actually going ahead was greeted with mixed results on the online community. Some were ecstatic that the movie was finally happening, while others, considering the comic to be unfilmable were upset with the news, considering the story to be unfilmable. Likely these people were concerned that it might shed a negative light on the comic, not unlike the way the Howard the Duck film was catastrophic for the comic book version.
As I type this, the North American premiere of the Watchmen movie is less than a week ago. I'm cautiously optimistic that I'm going to enjoy it, that it'll be worth the two decade wait. But one thing I'm sure of is, the source material will live on. I don't expect the movie to be exactly the same as the comic; that kind of juxtapositon is hard to pull off in a movie, and the movie would destroy everyone's bladder if it tried to match the length of the comic. But I think most people are smart enough to know that nothing's entirely word for word,k image for image.
But what if it proves to be very different in spirit to the comic? Back when howard the Duck came out, there were no trades of Howard the Duck material, and the trailer frankly was terrible. But in Watchmen's case, the trailer was compelling enough that comics of the graphic novel have been selling like crazy at book and comi shops alike. Furthermore, Watchmen has always sold well, pretty much every year winding up in the top 50 trades sold in comic shops each year. Unless the film is far worse than the trailers suggest, a lot more copies are likely to sell once the film has been released.
Therefore many people are going into the fil knowing the difference between it and the comic, even if they didn't know a year ago. More will follow. It's extremely unlikely that DC will create a new version with different words and art. The conclusion I therefore come to is that when the smoke clears, the comic will have lost nothing, and in fact will have gained a lot o new readers. Regardless of the quality of the movie itself, the comic will still be just as good as it's always been; it's just that more people will be aware of that fact.