Oh how things have changed for Barack Obama since he last appeared on “Saturday Night Live.”The Democratic presidential nominee’s campaign revealed Thursday that he would be making his second appearance on the program, for the season premiere this weekend. Also scheduled to appear is Olympic gold medal-winning swimmer Michael Phelps. Obama last appeared on the program in November 2007, making a cameo during an opening skit about a Halloween party at Hillary Rodham Clinton’s house, where she was portrayed as the presumptive president. As it turned out, Obama defeated Clinton for the Democratic nomination. Obama’s campaign said details of his new sketch were still being worked out.