As long as you use some green stuff as Lord Tsunami mentioned, you should be able to fit the new rider models on top of them without too much hassle.
Hicks: I think I got 9 or 10 of these still in their wrapping and another 6 or so assembled, was one of my last purchases back in the days that I never got assembled before I stopped playing. I wont use them myself as Im like the look of the dark elf cold ones better myself so Ill do a conversion with those. If your still looking for these old models, drop me an PM, depending on your location we might be able to work something out.