Complex sentence: On December 20, Tybor announced that Roback had become pregnant and wouldn't be performing with the band on their upcoming 2016 tour, and would be replaced by Joseph Garcia, who had previously toured with them.
Split into simpler sentences: On December 20 2015, Tybor announced that Roback had become pregnant and wouldn't be performing with the band on their upcoming 2016 tour. She was replaced by Joseph Garcia, who had previously toured with them.