The movie doesn't sound bad - at least it's not the typical Pokemon movie plot of some evil person trying to capture/control legendary pokemon and Ash and co. have to save the day. But I still feel like The Pokemon Company could have made this movie without completely changing the anime's continuity like this. For example, it could have been about adventures with Ash, Misty, and Brock that we never saw in previous episodes/movies that could have easily focused on Gold/Silver pokemon, Marshadow, and whatever else they want to promote right now. If you use the premise of "these are stories of Ash's Kanto adventures you never saw," that leaves the movie open to do pretty much anything without any need to change the established characters and events. Old fans would enjoy seeing their favorite nostalgic characters in new adventures, and new fans would enjoy learning about Ash's origins as well as seeing pokemon they're familiar with. Both groups of fans would be appeased, but by totally changing the continuity like this, they're going to alienate a lot of older fans.
I know that with nearly 1,000 total episodes, 20 movies, and numerous specials, the continuity of the Pokemon anime definitely has holes, but they felt like mistakes on the writers' parts while this just screams "we don't care about continuity at all." Again, I'm not saying the movie is bad because of this - heck, I'll probably like it when I get around to watching it. It just bothers me that they had to change it this way when, as I previously mentioned, I feel like they could have worked their current franchise promotion agenda into it without having to toss continuity out the window.