Clear the shader cache (both bade by video card and windows) and let the game to recompile them.
Use this code for windows cache (copy/paste in a .bat file)
@echo off
pushd %temp%
if not exist %userprofile%\AppData\Local\cache\ (goto _step2)
pushd %userprofile%\AppData\Local\cache\
for /d %%D in (*) do rd /s /q "%%D"
del /f /q *
popd
:_step2
if not exist %userprofile%\AppData\Local\D3DSCache\ (goto: EOF)
pushd %userprofile%\AppData\Local\D3DSCache\
for /d %%D in (*) do rd /s /q "%%D"
del /f /q *
popd
Similar code can be used for video card's shader cache but you didn't said what GPU do you have.