Being cable discs, the BB5's will be direct replacements for the Promax discs but don't expect a massive increace in performance, as the BB5's are still budget brakes.
With your current brakes, have you looked at the condition on the pads and cables?
The V-brake levers which worked with the Promax will work with the BB5's; the BB5's should come with new rotors and bolts, for the rotor bolts (these are a standard size for disc systems), as you are geting a new set with the BB5's, so why not use them
If you really want a performance increace, you really need to go hydraulic.