In a case where the state has previously announced it would waive the death penalty, and new evidence arises that makes the death penalty appropriate, can the state withdraw its intention to waive? Does the defendant have any right to the waiver previously announced?

You shouldn't characterize it as a waiver. It's an exercise of discretion by the district attorney. So long as the defendant is given notice in time to prepare, there shouldn't be any reason you can't change your mind.

Of course, once the jury has been selected, the decision would seem irrevocable.