There are 2 ways to do that:
1. Flash a fw tarball via Odin
2. Use heimdall to flash the new fw
Now you have 2 options:
1. Repartition to I9100 partition layout
2. Keep your existing partition layout
If you plan to flash again soon a Korean rom, I suggest you go with 2, otherwise 1.
If you choose 2 the steps are really simple:
1. Make an efs backup
2. Repeat 1
3. Reboot into Download mode
4. Fire up Odin or heimdall and flash the fw as pda
5. When it fails during factoryfs flash simply remove the battery and reboot again into download mode
6. Flash the kernel now only
If you choose option 1(which I think is the best) you will first have to uninstall Kies.
1. Reboot into download mode
2. Using heimdall go to utilities tab and where it says "download pit file" select a location to save it (i name it M250S.pit) and download it to your pc(not mac, we hate macs)
3. Reboot into download mode again.
4. Now using either Odin or heimdall flash the fw and select as pit the I9100 ext4 pit from 26 October 2011(on mobile currently so can't post the exact name)
In Odin tick also Repartition.
5. The flash will succeed with no errors whatsoever
6. Keep the M250S.pit in a safe.
If you want to go back to Korean simply repeat and substitute I9100.pit with M250S.pit
Sent from my GT-I9100 using XDA