If you come through London with any frequency, there's a repair place in South Kensington that replaces wheels on luggage. When my wheels broke on the Brussels cobblestones, I measured the wheels and bought the correct size of skate wheels (off eBay); they then put them in (because they couldn't source wheels for my particular make of bag, but they can for most). You could also ask them to modify the bag so you can easily replace the wheels yourself.

wg