The protaganist is Miyagi Ryota (the short point guard). Sakuragi Hanamichi is moved to be less of a focus, but still relevant. All other characters get a similar treatment.
The film literally launches you into the Sannoh match with little to no build up. If you didn't read the manga or watch the anime I feel most of this will make very little sense to you although it does tell a story that allows a newcomer to sort of see what's going on.
The entire Sannoh match is covered (the last 70? chapters of the manga), though a bit of it is glossed over from the first half. For example, if you literally didn't know about Sakuragi at all, though the film adapted the first very first shot of the game like for like where Miyagi and Sakuragi makes a face and the shot went in, to their shock, a newcomer would not know the relevance here.
The film is more dramatic/serious than what you would expect. A lot of the comedic elements are toned down.
The entire shifts between Miyagi's past and his life to the present day game. The other characters get their own as well, but not as prominent as Miyagi. There isn't much to say really about it other than that it just tell a story from another perspective. There some slight adjustments/modifications but nothing too damning.
The soundtrack is really good and builds up the hype well. I do miss the anime soundtrack though but it is what it is.
The SOUNDS are very realistic, gone are the loud and/or funny sound effects for most of the film.
I very much enjoyed the film regardless. Some people are definitely put off with the change in protagonist but it's ok I suppose.