Um... are you familiar with Gintama? The manga is excellent, but it's anime adaptation is off the charts. Sunrise does change things up, they usually add things in and it's almost always as funny as the source material. Gintama filler at it's worst is mildly amusing. At it's best, it can cause fits of violent laughter that send one falling out of ones chair.
If this is an animated movie we have absolutely nothing to worry about and everything to look foreward to.
If it's live action, it really all comes down to how they adapt it. As I said, it can be adapted. With the right premise, and the right cast, it'd be fine with or without a big budget production. Gintama's basically a sitcom, so there's a lot of wiggle room for them to get around the more outrageous and buget consuming aspects of the series. BUT there's not a lot of wiggle room when it comes to characterization - ultimately it's the characters that make the series.
Really, the biggest fear is that everyone will be out of character. The plot could completely suck, but so long as everyone's in character, it's still Gintama.