I was still on my first bulb at ~1500 hours when I switched to a Runco DLP projector so I never switched to the spare bulb. I didn't notice an appreciable dropoff in light output, but that might be because of gradual changes over time. I realize that the claims for these bulbs was that they would maintain most of the light output over the length of the bulb life (something like 90-95% according to the graphs I've seen on this) but that last 5-10% might be what is perceived as a bigger dropoff in light output. A lot of this perception has to do with the ambient light conditions of the room.
That said, I have no doubt that people who switch out the bulb once you get well past the 1000 hour mark will see an improvement in light output. In fact, if you have a spare bulb it would probably be a good idea to start using the spare bulb and keep the still useable original bulb for emergency service in case of a problem with the new bulb. As it turns out in my case I gave the VW10HT to my daughter and once she installs it, we'll do exactly that - switching out the old bulb for the new. During transport and storage I decided to stay with the old bulb in place.
As a side note: I never figured, when I purchased my VW10HT in early 2000, that over 3 years later it would still be performing nicely with the original bulb. It definitely exceeded all expectations. With the continual improvement in FPs with an accompanying drop in prices it's beginning to look as though some of the original concerns about bulb life and price didn't turn out to be a real world problem. Sort of like worrying when some of the computer companies switched from "lifetime" to 3 year warranties. 3 years is a long time in the computer industry and home theater equipment is an extension of that.
Moore's Law in action once more.
