A Last Will and Testament does not transfer property or assets until after the death of the maker of that Will.
But a person who is named Attorney-in-Fact by a Power of Attorney document can manage the assets of a Principal, so long as ‘best interest’ is applied.
If the assets described in a Will are no longer owned by the Will’s Maker after death, those assets are not part of the estate and are ignored at the time of distribution.
A person holding Power of Attorney does have the authority to transfer property of the Principal per the terms of the POA document while the Principal (the person granting Power of Attorney) is still alive, assuming that doing so is in the best interest of the Principal.
As such, so long as the Attorney-in-Fact named in the Power of Attorney document acts reasonably and in the best interest of the principal (i.e. without any self-dealing), any personal property items owned by the Maker can be transferred to another whether those items are listed in a Will or not.
The Attorney-in-Fact could obtain waivers from those named in the Will if he or she desires, but there is no specific legal reason to do so.