POA validity

Yes, its still valid, marrying does not change that. The Elderly man can assign someone new and revoke the old one,the spouse can't do it.

No one gets a POA or gets it changed. Only the elderly man can change it if he CHOOSES to change it. Generally it would be normal to want your spouse to be your POA. Discuss this with him, and ask his intentions. Also know that he must be legally COMPETENT to change the POA.

The new wife cannot change anything but her husband can if he decides to, and is still mentally competent to do so.

If he's competent, he can change it to whoever he wants.

If he's not competent, with documentation as required by the state law and the POA document, she can't change it to herself.