dont change all the lights at the same time. only change half of the bulbs. i.e. if you have the 4x65 aqualightk, then change one 10K and one actinic. if you change all the bulbs at the same time you will have a drastic change in the light output.
i would suggest changing a set early into the use of the lights, i.e. 3 months. and then at the 9 month mark change the other set. then you can use a 6 month rotation on replacing the lights... remember to keep track of the ones you replaced! i havent done this yet, but i was thinking to write the "in-service" date on the base of the bulbs (the white plastic piece at the base of the lights). probably use a sharpie marker or something similar.
~mike