Maybe I’m missing something, seems like an easy “paradox” to solve:

The legal regime recognizes the person as the same person, proven by the very premise of the question, that the older nobleman retains ownership over the funds. The wife should honor the promise because she made the promise with a person continuous with the younger nobleman in the relevant aspect (it is legal document). The fact that the older nobleman has different beliefs is irrelevant because the question already concedes primacy to the law, implicit in the law protecting the older gentleman’s right to dictate disposition of the funds. Someone enlighten me if I’m missing the point?

Now… if the younger nobleman disclaimed his old self and all prior inheritance (or had amnesia and lost all connection to his prior self), and built a new fortune somehow covered by the earlier promise, that would be a more interesting question. Then the person may not be continuous for ethical/duty purposes yet still continuous for legal purposes.