After reading the above response, I poked around a bit and by the power of google found this article:
written by Lennard Zinn, who knows a thing or two about the tech side. The actual question is more detailed and complex than mine but the gist of it is that, indeed the NDS bearing in the Truvativ setup is 2mm smaller in diameter than the DS, with the spindle diameter stepped down.
However, they use the same standard bearing on both the DS and NDS, which in turn is the same as on the equivalent Shimano/RF/FSA units - and presumably third party ones like King, Hope etc. The step-down is achieved via a plastic top-hat shim - which in principle can be removed from the old knackered Truvativ one and fitted to the shiny new 'standard' one.
Doubtless it'll be a little more involved than that, but I have a spare LX one in the shed so I'll give it a go over the weekend. Truvativ bearings don't have the best reputation (max. complement bearing + crap seal = premature failure) so it'll be good to see if Shimano fares better - or if it's actually possible to upgrade to King or Hope. Beyond that, now that the various diameters are known, it might be possible to retro-fit some decent bearings into the existing cups (eg Enduro bearings, like BETD do) and put it all back together.