I want to switch door knobs so I have a lock on my door that leads to my inside apartment within a house. I tried taking the old door knob off that as the "lock" but it won't come off. I put a piece of felt behind the flat part & it came off but the knobs seem affixed. Might this mean the knobs are glued or factory installed so the knob will not come off? If so then I'll just take the other knob off & buy a new knob with a lock to it. thank you. Minera Pearl