Yes, he can do this at any time, without the aid of an attorney, until he's incapable of making that decision. Though you may not approve of his revisions, remember that the laws that enable your father to draw up a durable power of attorney, giving you the right to make decisions for him, are designed to aid and protect him, not you. In many cases, circumstances change, and he may need to change his plans accordingly. Your father may have given your sister durable power of attorney, but if she moves cross-country, it makes sense to name a sibling who lives closer. Likewise, when people are in good health, they make decisions about end-of-life care that may be very different from what they want 10 or 15 years later, when arthritis or chronic illness has set in, or a spouse has died.
Although your father may not be able to care for himself physically at this point, that doesn't mean he can't make decisions about his own care and assets. In essence, this becomes a medical decision, rather than a legal one, and the criteria for judging his capacity to understand and make these changes are very basic. An examiner may ask him questions such as the following: Does he know who he is? Does he know who you are? Does he understand the document he's signing? When is his birthday? Who is the current president? If your father can answer appropriately, it suggests that he has the capacity to change or revoke any legal document. A lawyer may become suspicious, however, if the elderly person wants to name someone other than family who has come into his life only recently. However, revising a durable power of attorney for healthcare generally doesn't have as much potential for damage as changing durable power of attorney for finance.