Hi, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. I'm sorry to hear about this problem with Benji and his sudden and more frequent personality changes. I bet there is an underlying medical problem like a type of seizure activity, back or joint pain, or decreasing vision. All of these would definitely cause what you are experiencing, and all, if addressed, could resolve his sudden aggression.
I really recommend that you pursue this as a medical problem and have your vet examine him.
Do you have any comments or questions? I'd love to hear from you.