Once I had the new hub, I could take measurements of the new hub and and my rim and calculate the length my spokes will need to be. There are lots of spoke calculators on line, and Sheldon has several links. But I took the easy (lazy?) way out and made some direct measurements. First I measured from my existing hub to the rim. From the outer side of the spoke-hole to the rim. Then I measured the spoke hole diameter of my existing hub, again from the outer edges of the holes. Then I took the same measurement on the new hub. Took the difference and divided by two. That is how much shorter the new spokes will need to be.
With radial lacing, it's straightforward geometry. If I used another pattern I would have needed to use a calculator. You can see on my hub the spokes are longer than what a radial spoke would need to be, going further along the hub.