The top eight-ranked sides on the ICC ODI Team Rankings as on September 30, 2017 will qualify automatically to the 2019 event, while the bottom four will play in the 10-team ICC World Cup qualifier 2018 that will determine the final two teams in the line-up.
As a result, Afghanistan and Ireland will no longer play in the ICC World Cricket League Championship, the ICC's one-day competition for the leading Associate and Affiliate sides.
They will join Hong Kong, Namibia, Netherlands, Papua New Guinea, Scotland and the United Arab Emirates in the two-year competition, which is scheduled to start in mid-2015.
The new structure also includes a promotion and relegation system for Associate and Affiliate Members, with a challenge series to take place between the lowest-ranked Associate Member on the rankings table and the winner of the ICC World Cricket League Championship.
Ireland and Afghanistan will continue to play in the four-day, first-class ICC Intercontinental Cup 2015-17, along with Hong Kong, Namibia, Netherlands, Papua New Guinea, Scotland and the United Arab Emirates.
ICC Chief Executive David Richardson said: "Both Afghanistan and Ireland have excelled on and off the field in recent years, and this decision is a critical step forward to the ICC's aim of having more competitive teams in international cricket.