im pretty sure simply upgrading the firmware wont do. but i could be wrong..
there was a time when i encounted your same problem except i wanted the phone to read chinese. in order to do that, i had to flash the chinese firmware onto the phone, which cost money from where i live (melbourne, australia). seeing as it cost me money, i didnt do it lol

so in other words, upgrading the firmware through sony ericsson's update program wont work, you'll need to find some people who can flash the korean firmware onto your phone.