The UO.cfg file you are opening, are you locating the uo.cfg file via a search? What's the path?I only tried to make the change when the client was closed out completely.
What's the text editor being used to open it?
A quick checklist:
1) Disable "Hide extensions for known file types" - Very helpful where you have multiple files with the same name but different file extensions. (the 3 letters
2) Close UO. Backup your profiles under "C:\Documents and Settings\<your windows login name>\My Documents\EA Games\Ultima Online Legacy\" to somewhere, say a thumbdrive or c:\temp.
3) Edit your main character's UO.cfg file under "C:\Documents and Settings\<your windows login name>\My Documents\EA Games\Ultima Online Legacy\User Data\<your UO login name>\<shard name>\<main character name>
4) Save this, if an error pops up that says you can't save, google the exact words to see what the cause can be. You can also do a screenie and post it.
5) Test to see if it works by logging on your main
6) If it works, close UO and backup the uo.cfg in you UO installation folder (default uo installation folder is somewhere under "c:\program files" unless you changed it). Then copy the edited uo.cfg here.
7) Now the UO client will use this as a master copy if any profile is missing the uo.cfg file
8) Ensure you have made a backup of your profiles in step 2
9) Search under "C:\Documents and Settings\<your windows login name>\My Documents\EA Games\Ultima Online Legacy\User Data" for all uo.cfg files. Delete them. The UO client will recreate them for the edited uo.cfg in step 6
If you get stuck with a certain step, let us know, I'll expand on that step. Yeah, I'm lazy to expand every step hehe