I don't see why Yahoo would make any visible changes to Tumblr[1]. They care about the ad inventory; if they are smart they will leave the actual form of the site up to the people who invented it.
[1] although, as with Flickr, any changes that people don't like will thenceforth be blamed on Yahoo, whether or not it had anything to do with them
[1] although, as with Flickr, any changes that people don't like will thenceforth be blamed on Yahoo, whether or not it had anything to do with them