How? Your making a really generalist statement so provide examples to support your claim. Picards dialogue changed over the course of TNG as well. He went from sounding stiff and militaristic is the first 2 seasons, to a much more relaxed and thoughtful way of speaking in later seasons.
The "Romulans Only" sign and Picard's treatment of it and purposely escalating the situation, for example.
Whether you 'count them' or not is irrelevant. The point is that we've seen changes in Picards personality before. I mean we saw his personality change in TNG. He goes from being a dour stick in the mud during season 1 who hates kids and doesn't make personal attachments to his crew to a guy who plays poker with his crew mates and sees them as family. You might not like those changes, but the character does change because that is what people do in real life.
That's a good point about me "counting them which I'll get to in a minute.
I will say for right now that I think most of the changes for Picard in TNG were more natural.
Beyond that, even if you don't count the TNG films they are still fair game for the writers to inform them about Picard. Picard has a rebellious streak-he alludes to it in "Tapestry" and has that artificial heart to prove it.
About not counting them, yes, it's a bit hard to argue that he's out of character when he became more of an action hero for the films, which I believe was partially motivated by request of Patrick Stewart.
However, I think an argument is to be made for ignoring some parts of canon that don't work well. Take another franchise like Star Wars for example. Lucas retconned in "midichlorians" as part of the Force. Most fans dislike it and by the time Disney gets it, they completely ignore it. I think most would agree it's for the best.
"Tapestry" is an interesting one. I was kind of surprised back then to see Picard was this risk taking womanizer as a young man. And by starting a fight he learned to take risks? It sounds more like it would have made him wiser to NOT take
stupid risks.
But okay, Picard was a risk taker in his youth, by going along with a revenge scheme over losing a game of space pool, getting into a fight over pride, and getting stabbed in the heart. Well, that was way back then because by the time of him being Captain he came across as much more restrained, thoughtful and by-the-book, most of the time, and I thought he considered by fans in general to be more of the anti-Kirk.
So he was rebellious as a young cadet, restrained as a Captain in TNG, then more like Kirk in the films, and by the time he's 90, he's getting people killed because he started problems with the "Romulans Only" sign.
His change in First Contact didn't seem natural, it seemed retconned in order to fit the idea of what a typical sci-fi/action film should be. Just like Data saying "oh shiiiiiiiit" when previously we never heard anyone say "shit" in TNG, no matter how crazy the situation. It felt more like, "oh, this is a big time movie, so this is the obligatory comedic moment."