Call it a shakeup or call it a natural turnover halfway through the term, but the White House is preparing for significant change throughout its top ranks. Much of the movement, though, will involve new posts for longtime aides to President Barack Obama. Among the many moves afoot, Tina Tchen, who is responsible for working with outside groups, is preparing to move to the East Wing to serve as First Lady Michelle Obama’s chief of staff, according to two Democrats close to the White House.