After “a great deal of thought and soul-searching,” the producers of Lorimar’s “Midnight Caller” say they have decided to alter an episode that had drawn protests from San Francisco AIDS groups. Protesters said they were concerned that the episode, in which a bisexual man knowingly infects his partners with the AIDS virus is murdered, would increase violence against gays and AIDS virus carriers. In the new script, the man’s victim is prevented from killing him. “It was never our intention to visit pain . . . on any group,” said Bob Singer, executive producer of the series, which airs on NBC. Singer stressed the decision was not the result of pressure from AIDS activists who succeeded in shutting down production on the show’s San Francisco set last week.