Complex sentence: Following a U.S. tour, more lineup changes occurred, as Mayall replaced Tillman by 15-year-old Andy Fraser, who left within six weeks to join Free, to be replaced by Tony Reeves, previously a member of the New Jazz Orchestra.
Split into simpler sentences: Following a U.S. tour more lineup changes occurred, as Mayall replaced Tillman with 15-year-old Andy Fraser. Within six weeks, though, Fraser left to join Free, to be replaced by Tony Reeves, previously a member of the New Jazz Orchestra.