I agree with AtA, SoCal, AndyH, and Effen. Just my opinion:
1. Confirm that the plan did permit a J&S with a non-spouse (I'm skeptical; in 30+ years, I've never seen this, and I've seen a lot of plans). Your 2:48pm post indicates you have confirmed this.
2. If the plan is silent on permitted changes, then the answer is "no changes are permitted". That would preclude amending the plan to add the ability to change, since such amendment could not reduce any rights already in existence.
3. If the plan already permits a change (again, very doubtful), does the first contingent annuitant get nothing? I doubt that also.