The short answer is spokes can be reused if they aren’t damaged without compromising anything. I’ve got a mountain-bike wheel I’ve replaced the rim on twice that is still on the same set of spokes.
The long answer is a little more complex. When you take apart the old wheel, it is important to evenly release the tension on the spokes (say, one turn at a time) so that you don’t deform the rim in the process. When you build up the new wheel, the wheel will be just as strong as if you used new spokes, provided you tension the spokes properly. However, the individual spokes, if not installed properly, may fail prematurely from fatigue.
A spoke in a wheel doesn’t follow a straight line; it has a set of bends near the hub, a bend where it crosses the spokes that face the other direction, and a small bend where it goes into the nipple. A good wheel builder, using a few different techniques, pre-bends (or strain-relieves) the spokes, so that they are in the same shape absent the tension as they are with the tension. If you don’t do this, as the wheel rotates in use, the spokes are like a paperclip being bend back-and-forth, and eventually work-harden and fail. In a properly built wheel the spokes don’t fatigue in this manner – actually, if I get normal service life out of a rim without any spoke breakage due to fatigue, I prefer to reuse the spokes when replacing the rim. Since they didn’t fail, I take that as proof that they didn’t fatigue and were property strain-relieved. Why would I want to start over and take my chance on a new set?
Spokes can always be reused if the wheel is build rebuilt with the same type hub and rim, and provided you keep the spokes in the same position (left inner, left outer, right inner, right outer). Changing the rim type usually isn’t a big issue, but changing the hub flange shape could be. If you must reuse spokes in this situation, you may want to evaluate how different the shape might be and whether it would be possible to straighten them prior to lacing the wheel. Properly strain-relieving will be important.
If you totally ignore the bends in the spokes, reinstall them randomly, and don’t strain-relieve, worst case is that the spokes suffer from premature fatigue and break one-by-one. Breaking of a single spoke shouldn’t damage the rim. Properly tensioned, even if not property strain-relieved, the reused spokes will resist a “potato-chip” type failure of the entire wheel as well as properly tensioned new spokes.
You could re-drill the unicycle.com hub for the wider spokes, but you’ll need to property bevel the edges of the new holes. 12g spokes are 2.5mm diameter; I’d guess you’d want a 2.7mm hole, or about a #36 drill bit. Beveling the edges is left as an exercise to the reader. If you later wanted to rebuild the wheel with 14g spokes, you’d need to replace the hub as well.