change a dark hull color to white?

You would most definitely want to do this out of the water.

You could prime and paint over it. But I think I would sand most of the blue off down to primer. Then apply a light coat of primer before putting on the new white paint.

You're going to pay close to, if not the same as, if you were starting from scratch I would think.

It's not a problem to go from a dark color to a light color. The process is the same with any paint job. The cost is dependent on where you are as different areas have different labor rates. In New England you are probably loooking at a price of $12,000-$15,000.

I'm surprised to hear of anyone looking to change a newly Awlgriped hull to white. Is the allure of the dark hulls (with the associated costs & headaches) starting to fade?

If it is fresh Awlgrip it is sometimes possible to paint the white right over the existing paint. I don't advise this though. I advise that you sand it, prime it, sand the primer and apply the topcoat.

Awlgrip over Awlgrip

To all the above. It is not necessary to apply a primer over an Awlgrip Topcoat if recoating with an Awlgrip Topcoat. A scratch (sanding with 320 or higher grit) and shoot is all that's required.
I would, though, in the case of changing a Dark Green (or any dark color) to a white, apply a closed film of 545 primer. That means, sand the old Top Coat with 220 grit, apply approx. 75 microns of 545 primer, sand that with 320-400 grit, and apply the new Top Coat. This is a little extra work, but ensures that there is no bleeding through of the Dark color, which could change the shade of any White you might want.
Also if you are changing from the Awlcraft 2000 to the G+H Line (or visa versa) a primer is a must.
In thinking this, you could then sand the old Top Coat and put the primer and Top Coat of another Manufacturer (International Perfection, DuPont, Alexseal), and then have a, most probably, better Top Coat.
Hope to have helped!!

Hey Doeboy, No probs. If you have anymore questions, don't hesitate to ask. You can also PM me if it's more direct.
Rgds, Tom