Your dad reserved what is often referred to as a life estate. He will have the right to live on the property until his death. Your father can change his will immediately (but be certain to hire an estate planning specialist because the situation you're describing is likely to result in your brother's challenging the will). Your father cannot deed the house to you since you and your brother's already own the interests in the house following your dad's death (the "remainder interest"). Your father could gift you his life estate, but if he is living there it is unclear what the benefit of that would be, and it would have some adverse income tax consequences.

While it appears unlikely that your father can overturn a gift deed transferred years ago, you should consult with a local estate planner to consider the situation. Perhaps there might be some incorrect technicality in how the deed was recorded, etc.

There are a host of other issues your father should address given the situation you describe. You should be certain that he prepares and signs a durable power of attorney, health care proxy and living will. Given his concerns over your brothers, these issues are important to address as well. The estate planner your dad consults can help him with these issues.