It depends on how deep the sand bed is, but I personally like to keep a shallow sand bed (around 2" or 3") and every time I do a water change, I vaccume 1/4 to 1/2 of the sand bed.
If your sand bed is 3 years old and you have not aggitated the sand bed much in that time, then there is probably a LOT of stuff in the sand. I would definately NOT disrupt the entire sand bed. If you do decide to stir the sand or vaccume it, I would do no more than 1/4 at a time, waiting about a week between sections.
Just my thoughts... I'm sure others will chime in.
EDIT: Forgot to add that if you have a DEEP SAND BED, I would not touch it at all. That's just the nature of having a DSB, it doesn't stay white.
-Scott