Changing Research Groups
I would say that you would be better off going with the new faculty member. The question now is "How do I switch?" If this emeritus guy does email (which some of them are so old they do not. Although it sounds like you maybe are in Computer Science, so hopefully the guy uses email) I would write a brief email to him explaining why you are switching. This avoids a face to face if possible. Then unilaterally switch groups. I would take care of any introductions with the new advisor in person, however. 

Much of this also depends on things like if you share an office with people in the uninformed group, how large your department is (If you have 40 profs in the department, you can fade away. With 4 profs, not so much), etc. 

I switched drastically in what I was studying (Algebraic group theory as a MSci student to applied statistics as a PhD student) and there were a few eyebrows raised. But, it was ultimately my career. And people in my department came to accept it. It is SO much better to have a good research group than to have a placated advisor. Do not put yourself through that.