It feels the same as the TV series. Same studio, actors, art and character designs. (The TV series's second end credits sequence is a montage of clips from this film.) What's changed is the movie-quality animation, some plot streamlining and more emphasis on Okko's feelings about losing her parents. This is fairly subtle for most of the film, but it ultimately packs quite a punch.
This film was nominated for an Annie award and was submitted (but not nominated) for the Best Animated Film Oscar. I'm slightly surprised, but on reflection it is that good.
It almost fits into the TV series continuity. Not quite, but nearly. I watched this back-to-back with the TV series and got a sense of happy continuity. There are unimportant tweaks and tidyings (e.g. the roles of two writer characters get combined into one), but the main difference is that Okko takes much longer to befriend Matsuki. Same character arc, but this time it's played out over the whole film, not just a few episodes.
(There are also points where having watched the TV series pays off little touches in the film, e.g. the references to the god who blesses the inn's water.)
That said, though, the film's telling a very different emotional story. This is very much our heroine's journey. Matsuki is less prominent. The cute ghost friends and the supernatural are more central, including visions of Okko's late parents. (Most of these are probably just flashbacks and/or what's playing in Okko's imagination, whether she likes it or not... but that may or may not be always true.) If it weren't for the TV series, it would be possible to argue that this film's supernatural elements might all be subjective, including Uribou, Miyo and Suzuki. That would fit into late revelations (with no precedent at all in the TV series) that the ghosts' time on earth might be limited and that even now they might not always be visible to Okko.
Just seeing a truck on the motorway gives her a panic attack. And then there's SPOILER.
This is a warm, charming film. It's the best kind of children's story, i.e. one that's got teeth, but also an optimistic, humanist message. The ghosts can be funny. Okko herself is great. What's interesting to me, though, is how a film that got submitted to the Oscars is in most respects indistinguishable from its sibling TV series, which is a relatively obscure shortish-form anime for children.