Looks like I've solved this issue. I had an items.txt.bak file in the same directory. I moved it to another directory -- and new items.txt was applied!
I will do some additional tests, but at this moment it seems like the game reads every text file in the directory and applies them in some order, possibly redefining some values or giving one file a priority over another.
Update: I put the .bak file back in the scripts directory -- all the old properties and values appeared in the game!
That means, modders can alter default items and keep items.txt without making users to edit, delete or rewrite it, thus making possible easy uninstalling by just deleting mod files.
Further test showed that the game applies the file which names is "lower" in alphabetical order.
