I want to try a rim swap replacing a D521 with an X517 that I have laying around. Does anyone know if the spokes are of similar length, or if this is actually possible this way round? I would expect the spokes to be either the same length or need to be slightly longer for the X517, but all advice/knowledge is welcome.

I think it's probably worth getting new spokes, I know that this costs a bit extra, but unless theyre brand new, it's not worth the effort relacing a wheel to find that they start breaking after a short while...

.. it would be an extra tenner or so - all depends on how quickly you build wheels :-)