I recently replaced all of my T5 bulbs; I have two 36" TEK fixtures with 6 bulbs each. 3 D-D Aquablue+ and 3 Actinic+ in each fixture.
I started with replacing all of the bulbs in the fixture on the left. The next morning when the lights came on, I was able to make a visual comparison (sorry, no science) of the two. The biggest difference was how much more blue the new lights looked; this may be partly because two of the three Actinic+ bulbs run one hour longer than the rest of the bulbs and have 365 more hours on them.
Anyways, I then put an old Aquablue+ bulb in the fixture with the rest of the new bulbs and noticed that it looked quite yellow in comparison.
Based on my observations, I will continue to replace bulbs once a year. I didn't notice any significant "signs" that the old bulbs were faulty; the tank was doing just fine. The amount of "punch" that the bulbs lost in a year was most certainly noticeable.