If this were a consumer guide, I'd have the easiest job in the world. If you liked and watched the TV series Downton Abbey, you will like this movie. If you didn't like the series, don't bother with the movie. The only tricky area is for people who have never seen Downton Abbey but are curious. My suggestion would be to start with the TV show ... I'm not sure that the movie will appeal to someone who doesn't already have a history with the characters. But the film is more like a bonus episode than it is a standalone.
The differences are still worth noting. Primarily, Downton Abbey has always looked scrumptious, and it benefits from a big, wide, screen. (We saw it in Dolby Cinema, which wasn't all that noticeable for sound but which made scrumptious look even more so.) A couple of the new characters are interesting, largely because of the actors involved. Still, it's Downton Abbey, and no one acts too much out character, so it's a feel-good movie for the fans. Given the fairly conservative nature of the show, it is no surprise that there are no drastic changes here.
The similarities are such that I can cheat and cut-and-paste from what I wrote about the TV finale in 2016:
Julian Fellowes humanized the rich upstairs and the working downstairs, and he gave equal time to servants and royalty alike. The gradual progression of time meant we got a lot of talk about how we had to accept the future, which for the rich meant taking better care of their crops and starting new automotive businesses. But progress for the downstairs servants was always limited. Barrow was the most ambitious of the servants when the series began, and he was the most outright unlikable character on the show, as if wanting to improve himself was a bad thing. In the finale, Barrow got what he had always wanted: he became the butler. He didn’t become rich, he didn’t gain any power beyond the walls of the Abbey. But that was enough to fulfill his ambitions.
More problematic was Tom, whose social position leaped far beyond Barrow’s paltry desires. As the show began, Tom was the chauffeur, involved in socialist politics. He was quite the firebrand. Eventually, though, he marries Lady Sybil, and by the finale, he has long been established as one of the family, entrusted with Lady Mary to the managing of the estate, his socialism a thing of the past. His co-option makes the Crawleys seem liberal for their class, but they make no real concessions outside of accepting this one person. The class structure remains.
I could watch any random episode with at least some pleasure ... the dialogue was often entertaining, and much of the acting was excellent. But I had to turn off my brain, because if I thought about the show for more than five minutes, I always returned to the way Fellowes took the side of the upper class.
All of the above is true of the movie. Barrow experiences a personal moment that is heartening. Tom's past as a socialist is used for a weak and unnecessary side plot (this matters because in general, things move too quickly in the movie ... all of the characters get their turns, but for many of them, those turns are far too brief).
Of the newcomers, two stand out. Imelda Staunton, a veteran who I loved in Another Year, is the latest member of acting royalty to share dialogue with Maggie Smith. And Tuppence Middleton, one of my many favorites from Sense8, has a substantial role that seems to guarantee her presence in any future sequels.
I've gone on long enough. Once every three years seems about right to me ... I really don't need more seasons of this show.