The Betfred Championship will be increased from 12 to 14 clubs in 2019 in a knock-on effect from the decision to scrap the Super 8s.
The move is being made just three weeks before the end of the season and could mean that no team is relegated to League 1.
To get to 14, a minimum of two teams will be promoted from League 1.
The table toppers will go up automatically while teams from second to fifth spots will play semi-finals, with the winners of the promotional final also being promoted.
The final place in the Championship for 2019 will be decided by a play-off final between the team that finishes bottom of the Championship Shield and the loser of the promotional final.
Rochdale and Swinton were both staring at certain relegation until the new structure was voted in at an extraordinary general meeting of the RL Council in Manchester.
Swinton are currently second from bottom but are now favourites to avoid the drop and chairman Andy Mazey said: “This now puts destiny in our own hands and gives us a fighting chance to retain our Championship status.”
From next season all teams will play each other once home and away, plus Summer Bash, ending with a play-off series for the top five teams and the winner will be promoted to Super League.
The bottom two will be relegated to League 1, which in 2019 will consist of 12 teams playing each other home and away with the top team automatically promoted.
Teams finishing in second to sixth in the table will play-off to decide the second team to be promoted.
Rugby Football League chief executive Ralph Rimmer said: “As we have seen this season, the Betfred Championship and League 1 are strong, vibrant and highly entertaining competitions.
“We want to ramp up how we build value in Championship and League 1. This means more marketing and promotion including major focus games; streaming more games as part of a wider digital strategy and working with broadcasters to bring these competitions to a wider audience.”