Can a new POA be made with all prior POA's revoked?

Guestshopadmin wrote exactly what I would have written.  Revocation of previous versions is also standard for Wills.  A good attorney would automatically add those statements.

Whether the old ones are revoked or not, the new power of attorney, the LAST POA conferred, takes precedence over old one existing. However, it is safer, better, less confusing to revoke other POAs and this can be done in the creating of the new one. My advice is not to do online POA's. When you have them done their language is often more nebulous and uncertain, and their powers less well stated. See an Elder Law Attorney to create a new one. The attorney can examine the ability of the elder to do this conferring, and to know what he or she is doing.

The assignee (the person who is assigning the new PoA) must be competent to do so. The attorney will interview this person privately to assess their ability to understand what they are doing. FYI memory impairment does not necessarily disqualify them from creating a PoA because it doesn't impact their ability to understand what they are doing.

Yes, if the person is competent to assign a POA. There is usually specific language in a POA document that says prior ones have been revoked so there is no question about whether the new one is now the ONE POA document enacted