Been looking through the official website and it seems like there has been screenings in Japan already, after going through the threads in 2ch, this is what I found (potential spoilers here), also my translating skills are somewhat lacking so do please forgive if this is slightly inaccurate.
- Zangetsu Melon Arms doesn't appear at all in the movie (the poster was a lie)
- Sid didn't transform into Sigurd, he was attacked from behind when he was about to transform. Yoko manages to transform into Marika to defend against Idun when she attacked Yggdrasil.
- Ryoma gave Takatora the Watermelon Lockseed, saying that this is his replacement for his lost Suika lockseed (stolen by Micchi) and it's a prototype to be tested.
- Idun manages to steal the Melon Energy Lockseed so Takatora is forced to fight as Zangetsu, once he manages to retrieve it he turns back into Zangetsu Shin.
- The Ringo Lockseed has side effects and causes pain to the user. Ryoma calls it a failure just like Yomotsu Heguri, although they both can achieve a limited amount of control of Helheim.
- Idun is killed by Duke eventually.
- In Baron's part, Bravo and Knuckle fought Tyrant but lost.
- Tyrant's Dragon Fruit in the end went berserk which turned Alfred into an Overlord like kaijin.
- Yoko gives Kaito the Ringo Lockseed and just says it's a present from Ryoma.
- Baron defeats the Overlord with the Ringo Sparking Rider Kick.
- The Ringo Lockseed is destroyed after Baron finishes with it.
- Overall the Riders that appeared are Baron, Zangetsu, Bravo, Knuckle, Zangetsu Shin, Duke, Marika, Kurokage Troopers, Idun and Tyrant.
There are many other details about the character backgrounds but I can't make too much out of that, other than I think Kaito's father committed suicide.