LG G3 stock rom change

[AC Question]

Hey guys,
I just purchased the LG G3 F400L (The Korean) from amazon and I found alot of limitations.
Im not from korea so Im not using a korean sim.

#1 every time im making a call, a messege keep puping up asking if I want to call local or call to korea. (happend only with numbers without an international pref)
#2 I cant make the LTE works, this option is just blocked (gray)

i think all those limitations are in the ROM level (ver F400L10k) so I was thinking of swithing the ROM to the D855 ver.
Do anyone know if its possible and what the risk ?

thanks alot !!