Crossing over from Ativan to Valium

Can someone please explain to me why you would get withdrawls from switching over to Valium.  They are both Benzos, so why would there be a withdrawl?

Thank you in advanced

Ativan builds up and is eliminated pretty quickly.  Valium builds up and is eliminated very slowly.  So when you stop Ativan and start Valium, the Ativan is eliminated pretty quickly, but the valium builds up slowly.  So for a week or so you end up with less benzo in you than usual.

Hope that makes sense.

There are strategies you can use to circumvent this effect.  Ashton has you do it in 4 stages which reduces the coverage gap.  Alternatively, one can adjust dose timing and amounts during the crossover, but that gets a little trickier.