As with all versions of software, the DirectX API will prevent you from "downgrading" to a lower version. In fact, there is no real way of "uninstalling" DirectX either.

Few points regarding simulators: they are not all built the same. My Reflex XTR did not require a good graphics card (even though I had a decent card then). About the same time I bought the G5 I also upgraded my PC, so I obtained 2 x ATI 4550HDs 512MB each in Crossfire and an I7 processor so my PC doesnt struggle with anything. G5+ also requires 1GB RAM, 4GB HDD space and graphics card with per-pixel shading support so the requirements are quite heavy. You need sturdy hardware for G5 and having got that, it gives you very good results. So if you have a older PC you may want to factor in PC hardware cost into the sim decision

Ameyam