I pulled the classic UI out from the original CD months ago - the issue here is the serverside software which isn't based on 'classic'. Lots of features have been turned off (and even more can as well - hotbars 2 to 4 etc) if the devs can find the code to do that.
The XML code is completely different from what it was for classic and I don't think it would even be possible to go back to the first style at all. Like someone said there is a classic looking UI out there but its no where near perfect unless you like playing at 800x600/1024x768 crappy resolution because of the way the windows are put together.
Here are the bmp files, good luck:
Link