The film had originally been set for a December 2021 release.
Dame Maggie Smith, Hugh Bonneville, Laura Carmichael, Jim Carter, Michelle Dockery and Elizabeth McGovern will all be reprising their roles.
While Hugh Dancy, Laura Haddock, Nathalie Baye and Dominic West are joining the cast.
A teaser for the new film was shown at the CinemaCon event in Las Vegas, reportedly including a snippet of a wedding.
The hugely popular TV series aired on ITV from 2010 to 2015 and followed the fortunes of the aristocratic Crawley family and their downstairs servants at a Yorkshire country estate.
Show creator Julian Fellowes is returning to pen the script while Simon Curtis, whose credits include My Week With Marilyn, is on directing duties.
The first movie, in 2019, followed a royal visit to the Crawley family and Downton staff.
It was well-received by critics as a decent, if unspectacular, addition to the series.