Something happened, perhaps when no one was home, but something changed between them.
I agree with allowing Daisy to reclaim her spot as top dog. She shouldn't have to give that up and as she's getting older she is becoming less tolerant of not having her place in the pack respected.
It's not the same but we have a dog and cat who might as well be glued together they're so attached to eachother. Still, I feed the dog first, she goes out the door first, if they get treats, she is first.
Take a little time to work on it, and consult your vet and/or a trainer for additional tips. You are right, neither deserves to live this way. I wouldn't give anyone away until you've tried everything. To many people and animals would be hurt if you didn't.