The knockout round will be played in March and create continuity between the group phase ending in November and the Finals in June.
League A group winners and runners-up will participate in home-and-away format quarterfinals, with the winners qualifying for the final four.
The third-ranked team in League A and the runners-up in League B, as well as the third-ranked League B side and the League C runners-up, will play a home-and-away promotion and relegation playoff.
"By introducing the new knockout phase, teams will be given even more opportunities to progress while keeping the same number of games within the international match calendar," said Uefa President Aleksander Ceferin.
In other changes, the European qualifiers for the European Championship and World Cup will be more consolidated, with teams being drawn in 12 groups of four and five teams.
"The predictability of the European qualifiers has also been addressed and tackled, with a fresh new format that will offer all the teams an equal chance to qualify for major tournaments," Ceferin added.
"The new format promises to bring more excitement to Europe’s national team football competitions that continue to grow from strength to strength."
The new formats for these competitions will come into effect in September, 2024, Uefa said in a statement.
The 2023 Uefa Super Cup has been moved from Kazan, Russia to Athens, Greece.