Day 6: Booyah!
First: The Un-rooting
Required: Kies, Odin, Triangle Away, a flashable version of Clockworkmod, a flashable version of YOUR ORIGINAL CHINESE FIRMWARE
1. Backup your phone data
2. Shut down Kies
3. Do Triangle Away
4. Boot to download mode and use Odin to flash YOUR ORIGINAL CHINESE FIRMWARE
5. When the phone reboots, do a factory data reset
And bazinga!, your phone is now just as you bought it. To confirm, take a look at Settings > About device > Status > Device status. (If it still says "modified", then something went wrong. It should say "official" or something similar.)
Second: Teh Flash
Required: Odin, a flashable version of clockworkmod, ANY i9300 FIRMWARE YOU LIKE! (I chose I9300XXELKC, because it's Android 4.1.2 and the fact it's intended for Poland makes no difference.)
1. Boot into download mode and use Odin to flash your new firmware (PDA only! Not CSC)
2. Watch the flash fail on the last "write" operation
3. Boot directly into download mode again
4. With Odin, flash clockworkmod
5. In the clockworkmod recovery menu, select reboot device now.
6. Be amazed.
As of now, my phone has Android 4.1.2 installed. I added a gmail account and the phone registered itself with google. I have Google Play, and did browse. My APNs didn't change. The number of installed languages increased vastly (from 3--Chinese, Korean, and English--to [lots] including 5 kinds of Chinese and 8 kinds of English).
CAVEATS:
I screwed with my phone quite a lot before un-rooting it. If you've never rooted your phone, you may not experience that final write fail, and steps 3-6 with clockworkmod would be unnecessary.
But what that write fail was, I don't know. According to Odin it means the flash failed. According to the phone, the flash succeeded. As of this moment I have once powered the phone off, then on: it booted normally. I have played with it for about ten minutes. Is something crucial missing? I don't know.
Also, I don't know if clockworkmod is necessary--maybe you can recover using Kies instead.
And, since putting the new firmware on the phone this way means the phone counts as "modified", it will not register for software updates.
Lastly, the biggest caveat of all..... THIS IS NOT A DETAILED GUIDE! Consider doing some research.