Using the Blizzard's Interface kit, we can change various UI textures by simply replacing them with our one customized ones by mimicing the folder tree and names such as:
Blizzard's Interface kit tells me that the Loading screen textures are located in the following internal folder:
So to test if I could change a loading screen, I took the one for Eastern Kingdoms (LOADSCREENEASTERNKINGDOM.BLP) and named it according to the one for Kalimdor (LOADSCREENKALIMDOR.BLP) and placed into the folder that I created like the one above.
However the replacement did not occur as I had hoped, logging into Orgrimmar yielded the regular Kalimdor load screen.
Using one of Blizzard's existing BLP files ensures that all attributes are correct, but I tried making a customized BLP myself, which did not work either.
What am I missing here? Is this an UI modification not allowed by Blizzard?