The change, agreed following meetings at Lord’s, has been made to bring an “improved shape” to the domestic season.
The ECB announced that, from 2017, there will be a top division of eight teams and a second tier of 10 counties.
In order to achieve that new balance, counties in the Second Division will be playing for just one promotion place this season — with two relegated, as in previous years.
The Championship will be run in segments of fixtures, pausing first during April and May for Royal London Cup matches, leading to a July final.
Four-day cricket will again take a break during July and August, when the NatWest T20 Blast, retained in its established format, will take place.
ECB chairman Colin Graves welcomed the alterations to the domestic calendar.
“The changes for 2017 will be good for fans, players and our international teams,” he said.
“The season is easier to follow; the blocks help players focus on specific skills, and there’s a better balance across all three formats.”