I don't think they can outright admit they're forged. They'd have to reveal their source, and that might be even more devastating to them.
I think they're going to insist that the authenticity of the docs is ultimately "unknowable," i.e., reasonable people can disagree, but that they contain "larger truths" that must be addressed. I said from the beginning that someone's gonna argue that these docs were based on handwritten notes or, better yet, conversations that existed 30 years ago, and I think we're already seeing that.
BTW, my theory is that Rather's "unimpeachable source" for the docs is his daughter, who is active in Texas Democratic politics. She got them from Ben Barnes and vouched for their authenticity. That's why Rather is digging in on this. Just a theory.