Durham head coach Jon Lewis leaves the club after a "review of cricket operations" at the Division Two side.
The England & Wales Cricket Board confirm their new competition in 2020 will be 100 balls a side - made up of ten 10-ball overs.
County champions Surrey will start their defence of the title in April 2019 with a home match against Essex.
Experienced Sunderland-born seamer Chris Rushworth extends his contract with County Champinship side Durham until 2020.
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Durham's former England captain Paul Collingwood says he was "a blubbering wreck" when he told his team-mates he was retiring.
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How to get into cricket - physical exertion with a chess-like element in matches which range from half an hour to five days.