It’s not a boundary you’ll find on any map, but any Manhattanite will tell you that the stretch of 96th Street east of Central Park is one of the brightest and longest-standing borders in Manhattan real estate. With the ultra-prosperous Upper East Side on the downtown side and East Harlem on the north, 96th Street marked a long-standing class boundary as much as a dividing line between two neighborhoods. But, thanks in large part to a host of new luxury rentals on the upper reaches of the Upper East Side, the bright line along 96th Street is looking notably less distinct these days -- and two culturally rich neighborhoods have started to look more alike. Call it the upper Upper East Side if you like, but the changes along the northern part of the Upper East Side are unmistakable and undeniable. The Upper East Side, in other words, is moving uptown.