Why would you even want to do that? It was popular with Grados because for starters the chassis design makes it easy; and then to some it corrects some other flaw they find in them where correcting it with a different headphone may lose the kind of Grado flavor they like, for example using a longer chamber to get more bass,or a spacer or an adapter for different pads for comfort. Also, technically any headphone cup can be swapped out, it's a question if you have the right tools and skills, but in some cases the plastic housing doesn't have too much resonance or produces a good enough response; or they're harder to do, and will make the headphone look worse rather than how good Grados look with wood.
I'm guessing he means the pads, not cups.
that whole issue is WAY overrated IMO. Go and read the reviews from X1 owners and you will find TONS of praise & satisfaction for them, with many stating that they sound better than headphones that retail for over $500.