Complex sentence: The lease of the first Boeing 767 seems to have been replaced by an Airbus A320 in 2006 and then replaced again with a Boeing 757 in early 2007 and yet again replaced with a DC-9 in late 2007, ending up with an MD-83.
Split into simpler sentences: The lease of the first Boeing 767 seems to have been replaced by an Airbus A320 in 2006 and then replaced again with a Boeing 757 in early 2007. It was again replaced with a DC-9 in late 2007, ending up with an MD-83.