Complex sentence: All those years she represented England and Great Britain at international competitions, but between January 2006 and March 2008 she competed for Australia, winning the 200 m backstroke event at the 2006 Commonwealth Games.
Split into simpler sentences: All those years she represented England and Great Britain at international competitions, after which she transferred her sporting allegiance to swim for Australia. From January 2006 to March 2008 she competed for Australia, winning the 200 m backstroke event at the 2006 Commonwealth Games.