Originally Posted by
Retro Grouch
Unless your new rim is a near duplicate of the one that you have, your existing spokes won't be the right length.
That's correct. As I build (and repair) more wheels, I'm learning that certain components allow greater margin of error than others. For example, deeper, double wall rims can safely tolerate some spoke threads clearing the top of the nipple. There's a valid concern that you can bottom out the threads before reaching the desired tension - so what I've done is have an LBS with a Phil Wood spoke machine roll an extra 5mm of threading. This has saved me a few bucks and made some otherwise tricky repair scenarios possible.
Also, in a pinch I have laced a front wheel 2x instead of 3x to make use of shorter spokes that I had on hand. None of these suggestions are optimal, but they can be useful improvisations.