Katt Williams' Replacement -- Audience Wasn't Feeling Him

The guy who will be headlining -- in place of Katt Williams -- says the comedian was trying something different Saturday night at the Improv in Ontario, CA ... and the audience just wasn't into it.

As TMZ first reported, Williams became belligerent with the audience and was asked to leave the stage -- and cops were eventually called.

But Maronzio Vance, who opened for Katt that night, thinks fans were upset because Katt wasn't his usual hyper self. "He was mellow, he was chill. He hadn't been on stage in two years."