Recasting is hearsay and nothing more. This script isn't even in the written stage, let alone the completely finished, completely funded and ready to shoot. A lot (or in Henson's usual case, nothing) can happen from that point. Unless this were to be filmed when Muppets 2 were filmed, I doubt Steve or Dave would be unavailable. And there's always looping dialogue if needed.
The only characters that would need a recast so far are ones played by Jim or Richard. And none of their characters were the main 5. If we have the Gorgs in the film, we'd need a new Junior performer (I nominate David Rudman)... but I doubt we'd see much of Cantus or Convincin' John.
Now, I can see Matt and Jerry team up Sesame Street style. If worse comes to worse, Matt's been doing great in Jerry's roles.
And I'm sure the rest of the gang would be willing and ready to perform those characters once again.
So unless anything is definite, there's nothing to worry about.