Because those are VERY different, and it will be either very hard or nearly impossible to transfer UE4 rendering features onto DX9 hardware.
Instead of DX9, you’ll really want to stick with OpenGL 4. OGL 4 is pretty much equivalent to either DX10 or DX11 (depending on OpenGL version), and you’ll have all those extra features like geometry shaders on XP. There’s no point in going back to DX9.
Getting your GL-based project to run on WinXP should require less changes either way.