Okay, first things first, you're getting new Style Five music in this movie and the art and story are good as always. Second, don't watch this as a replacement to the anime series because it isn't.
However, unlike the first movie, I do actually recommend that you skim through this movie. Let me explain: the first movie and this movie are both compilations of Free!: Eternal Summer. Each movie shows different scenes to tell the same story, but you can get most of those scenes by watching the anime, so it's kind of useless to watch the movies. Except for the fact that you're gonna
get a few brand new scenes in both movies. The first movie's extra scenes don't really add much to the main storyline; they're just there as Easter eggs. This movie, though, does feature some content that benefits the main story. You'll also get more extra scenes in this movie than you will in the first one.
I think this is because the first movie essentially followed Haru and Iwatobi while this one followed Rin and Samezuka. Since the original story is mostly about Iwatobi, there isn't as much content on Samezuka so they had to add more to this movie to fill the time. They do rework the scene where Haru asks Rin to come to Iwatobi. Instead of him asking Rin after a tournament, he asks Rin over the phone. Not that big of a deal, but it's still different.
The rest of the scenes either show you something that the original story just told you about or they add something completely different to the story. For example, in the parent story, Makoto says something like "I discussed the issues about Haru with Rin." In this movie you actually get to see that conversation. At the end of this movie, you'll see Sosuke tell Rin about Ikuya, warning him to watch his back should he ever race against him. That's one of the main things I think adds to the parent story because in the current storyline (Free!: Dive to the Future), it's very possible that Rin may end up racing against Ikuya.
Some things you need to know before watching this move: It's a good idea if you've seen at least Starting Days before watching this. There will be more Easter eggs in here that won't seem as cool to you if you haven't seen Starting Days. Also, I'd recommend watching Take Your Marks before this as well just because this movie will lead into it and it's nice to know where it's leading to.
Again, don't watch this as a replacement for Free! For one, this movie and the first skip past the first season, so you need to watch that anyway. For two, it doesn't have the little details in it that the actual series does, so you'll miss good bits and pieces. Also, this movie is mostly about Samezuka, Rin, and Sosuke, so just watching this movie is a bad idea anyway.
However, if you haven't seen Free! in a while or if you want to catch up on it before watching the third season or Take Your Marks, these movies are pretty good tools to use to refresh your memory and even get a few new insights on the characters. Basically, I just skimmed through the parts that are already shown in the series, and I think it was pretty effective. If you like Free!, at least take the time to watch the extra scenes because they're worth it.