Well that's quite strange, I had done the same like a hella long time ago, and this issue didn't happen? Did you just replace the textures or the other files as well? Because I replaced the textures only and my FH turned out fine.
Understand that FH+ is a larger game with different material files, etc, therefore it will indefinitely alter your original FH which is smaller, and with more or less, simpler material files, you get what I'm saying? Perhaps just re-install and replace the textures only, should work. C: