Should I have my mother change her beneficiary to me since we live together?

I agree with Eddie. It's not likely he'll cooperate, and since your mother has dementia, she likely can't do it herself (if she were cognitively okay, she could make the change without his approval). But you can try.
Good luck to you,
Carol

K:

Call and ask him to come over to discuss what's in Mom's best interest(s). Although medically he has a heart, you probably think he doesn't care at all. But appeal to it anyway, tell him that you need him. There's nothing like a damsel in distress to make a guy come to the rescue. Most will help; others will leave her tied to the train tracks. But you won't know until you have a candid chat with him about what's best for Mom and how important his cooperation is.

When she's gone, then you can reconcile your differences. ... Or not.

Who is "him"? Husband, son, who? Maybe a part of your queston has been dropped. But talk about buying a law-suit. Since mom is suffering from dementia, it is likely that she does not have the mental capacity to to change the beneficiary o,r if she is like my wife, she may not even be able to sign her name. The only answer that I can see short of some legal expenses is for you to talk things over with "him" and if that cannot be done sometimes one just has to face facts.

It is troubling to see a woman who is the center of such a difficulty and who cannot participate in forming a solution. It is impossible to make a clear answer without hearing "him's" version and, of course something from mom. BTW, who, if anyone is paying the premiums on the policy?

Him is my brother