Nobody has it over Liv Ullmann when it comes to changing, but even Liv's resilient psyche might have felt a little harried after her most recent role reversal: from Anna Christie, a boozing lowlifer in the O'Neill play of the same name, to guest of honor at a black-tie and long-gown reception at the Plaza Hotel in New York. The party was thrown by Anna Christie producer Alexander H. Cohen, who picked up the tab for the pea soup, cold salmon and California Chablis. Celebrity-watchers at the door risked whiplash as 580 of the great and near-great from Alda, Alan (black tux) to Vanderbilt, Gloria (white gown) poured in. True, the Aldas did turn heel at the sight of the crowd, Al Pacino, Ellen Burstyn and Lily Tomlin didn't show, and Shana Alexander and the John Lindsays left before dessert. But such couples as the Harry Belafontes, the David Susskinds, the Bert Bacharachs Sr. and the Sander Vanocurs toughed it out to the early-hours end. Despite the crowd and its standing ovation at her 11:30 entrance, Liv seemed a bit removed from it all. Best friend and fellow Berg-maniac Bibi Andersson, to whom actor Kevin McCarthy was paying close attention, said of her quiet table-mate: "I don't think Liv knows most of these people."