So how to change it back is what I am to say. But before that I will try giving a brief intro of this interesting file.
If you open the CSC_I9020_EUR.tar, you will see the dgs.img, which is totally a new stuff at least to me. To view it, actually is easier than most of you think. Notepad is Okay.
The old bootloader doesn't read this file, so it display nothing, even when I flash a wrong one, the new bootloader cannot boot the phone up while the old one can, which troubled me for couple of hours.
（If U guys are interested in the process I figure this out, plz move to http://forum.xda-developers.com/showthread.php?t=987179 thread to see the comments, all by me）
I firstly just tried to edit it through the notepad, but failed. So I read many threads towards editing img files(like recovery or system)
Here is the way how(Finally) :Use any hex editor like "Hex workplace" to open the dgs.img, and change the EUR numbers to 54 4D 4F, two places in all (Where they were 45 55 52). And press save. And it's done.
Two ways to flash to the phone:
You may use cygwin to make a tar file to Odin it to the phone, like putting it in the C:/cygwin/DGS, and use "cd /DGS" command to access to the folder, and then type "tar -cvf CSCNEW.tar dgs.img", and this tar file created is an Odin-able one. (I tried 7-ZIP before this, but the tar is not usable) *CYGWIN is a stuff building a linux environment under windows system. You can download from www.cygwin.com, and it's free.
Or you may just do the thing I did, move the new dgs.img to your fastboot folder, and boot your phone into the bootloader, and type
"fastboot flash dgs dgs.img"
Okay, the TMO is back now.
The reason why we just can't edit it in notepad is because this will change the 00 between the two EURs(Or TMOs we want) to 20, like it was originally 00 00 00 00 00 00 00 00...and now is 20 20 20 20 20 20...Then this is the point.
P.S Although there shouldn't be any problem if you actually obey the instructions. But if you mess it up, just odin the originally csc file(just choose CSC_I9020_EUR.tar only) to the phone back. You always have a second chance.
One thing is when you mess it up, the download mode may look very un-normal(totally almost black screen with a little colors on it), don't worry, it is still workable and can be recognized by the odin.
Like always, try this at your own risk. Personally I don't think this file acts much role in the phone, just a line of code.
~GT-I9020 TMO TMO 2010-10-14 - I9020TEST 0000 0000 .. KB100D00WM(I9000) - -
====(DON'T USE THE CODE ABOVE DIRECTLY, JUST FOR EXPLAINING ONLY)=====
So don't be too serious to it.
At last, 世界人民大团结万岁！~
Cheers and Good Luck!~
(Sorry for my poor English since I am a 21 year-old guy from China...)