No, it's a fascinating subject really, but we've more or less settled on a conclusion based on the evidence of empirical data. There is no point replacing a bulb if it hasn't blown. If you can see the light then it's useful. One would have to do very precise measurements in order to tell the difference in plant growth between a year old bulb and a new bulb, and even so the difference is likely to be related to many other things such as CO2, flow and so forth. We already know that spectrum changes are irrelevant so why spend money on a new bulb when the old one works fine until it goes kaput.
Here's some empirical data for you:
I replanted some P. helferi in this section of the tank. These are sitting at a distance of about 24 inches from the bulb. The bulbs were about 15 months old at the time:
This is the same scene 8-9 weeks later. All the extra growth were strictly from the originals in the first photo pruned and replanted. In fact there was a lot of growth. You can see blyxa starting to muscle in on the right.
Here is the same scene just above the P. helferi a few weeks after the second shot. By this time the bulbs are about 18 months and a couple of them went kaput so I replaced them. Now, please explain to me why I need to replace bulbs before they pop? Spectrum change? Yeah, maybe, but so what?
OK, so those were slow growing carpet plants. How about the effect on stems? Here is the other side of the tank. The tired old bulbs on this side were also around 15 months old and had plenty of miles on them.
Eleven days later with "throw away" 15 month old bulbs. Can you see a difference? So people really need to get real. Test the throw away theory for yourself. See what you can grow with old bulbs and compare with new bulbs.
Four weeks later the red stems have finally caught up. There were several trims during the time. Is this enough growth performance for you? I'm not throwing away my hard earned cash for some myth. That's for sure.
Cheers,