If you're referring to "Ark of Truth" it's just a director's cut. The new movie "Stargate Continuum" is a totally new concept; Ba'al goes back in time and keeps Stargate SG1 from ever being formed. I get the idea it is more along the lines of how the series was when Richard Dean Anderson was in it.

It's going to be like Star Trek. Now that the series is over they'll make movies until we quit buying them.

I don't remember any plot changes in "I am Legend" either. Someone who's already seen it might be able to answer that.