There was a lot of instrument switchup on the Chicago X album. They also had five lead singers; Lee Loughnane and James Pankow sang lead on one song each. They brought in a lot of session musicians of course, for the strings and French horns.
Up to that point, Robert Lamm was their main songwriter, but by then, he was so deep into drugs, his output greatly diminished.
And without Nixon in the White House, he couldn’t write songs like “A Song For Richard And His Friends” anymore.