I have built a few wheels now and am slow but more confident. I had a nice set of Mavic rims that had Triomphe or Victory hubs. I took the wheels apart one at a time, one spoke at a time and relaced them using the existing spokes with Super Record hubs. They are great and I have had no problems.
Building wheels is, or was, one of my unresolved childhood issues. I could never do it as a kid. I did build one, a steel rim wheel for a cheap Gitane when I worked at a LBS, but never more than that. So, it became a personal challenge.
I have also built a pair of Weinmann Concave rims with Campy large flange Record hubs. I had the LBS, a nice and patient one, measure the rims and calculate the spoke lengths. It all went without a problem.