You can. There is no visible difference between DX11 and DX9 really.
DX11:
- Access to MSAA, few people seem to use it due to its horrid performance
- TINY bit better water, difference is negligible
- Access to HDAO
- People claim there's a lighting difference, I've seen none
DX9:
- Better performance
- Can add AO through the config, some settings almost on par with HDAO
- Better draw distance on trees
- Can use SMAA / SweetFX
- Allows proper Alt + Tabbing. DX11 does some weird windowed mode that works the GPU
Outside of that, they're identical. This game has the worst DX11 implemention I've seen
Unless you are desperate for MSAA, I really can't recommend DX11.