how no Sidney will change things
Because since it seems like there was a script a while ago, Sidney was assuredly in it, and as the "lead," even if more screen time goes to Sam again. And so presumably there will have to be a rewrite that doesn't include her, if that version didn't already exist.
So it'll be interesting to eventually hear how the two versions differentiate, probably after the final product releases. For example, maybe in the version with Sidney, Kirby is the opening kill. But without Sidney, they retool it so that Kirby is alive and present through the whole thing and makes it out alive, to be the new Sidney, like a forebear over Sam. Stuff like that.
And now I guess it won't make sense for McDreamy to return.