CHANGE comes slowly to the Yankee Doodle Tap Room, where we have been dropping in for more than 40 years. The bar, with its well-known Norman Rockwell mural of Yankee Doodle riding on a pony, was moved to the middle of the room during a renovation several years ago. Last year, five large screen HDTVs were installed near the bar, where they hang incongruously close to the magnificent stone fire place. Another recent and notable addition is the photo of Michelle Obama among the senior pictures of other eminent Princeton graduates — Donald Rumsfeld, Ralph Nader, Bill Bradley — that decorate one wall.