Any meaningful change will be so jarring that the dyed in the wool users will balk at it, kicking up a storm of protests and it will not serve to bring in new users (they come in not for good UI but for the network effect - but they don't use it and leave because of the UI).
I honestly can't believe how insanely horrible twitter UI is - and it seems to be something that all twitter users have agreed to never mention. It just leaves one breathless after it opens the FOURTH modal window on top of the previous three that it had already opened.
On the bright side of things, for anyone teaching UI, twitter is a fantastic example of what NOT to do.