To add, how many times have we've seen changes over the course of the Spider-Man films in terms of characters? We had Mary Jane in the Raimi films, but she was largely spliced with Liz and Gwen. We've had Gwen in the 3rd film, but she was kinda-sorta MJ..at least the modelling aspect. Move onto the TASM series and we had another mix of characters to create their Gwen Stacy.
Peter Parker certainly isn't exempt of this, either as Tobey Maguire was largely a 1960s caricature in the original trilogy who never really grew with his alter-ego. Andrew Garfield was deemed, too hipsterey or too James Dean-y for Peter Parker. And now Tom Holland, while having the bare necessities as did the other two, is also a different Peter Parker in terms of his reckless and naive attitude.
Point is, if they hadn't taken from page-to-screen before with characters 100% in the past, why would they really now? Heck, even the MCU heroes which are often praised the most are exactly 100% portrayals in terms of character on page to character on screen. I'm not a fan of the decisions sometimes, but it depends on the direction of it, too. If they're really going with the John Hughes aspect, then having a transformation of a character from start to end isn't the worst thing in the world..at least to me.
It's not Mary Jane Watson everyone knows, but neither was Raimi and that series got off okay. TAS had a mix of MJ along with Gwen and that did alright. TNAS also changed the character but she wasn't a bad Mary Jane by any means. SSM changed Gwen and nobody took issue with that.
..Then again, this film is already changing characters or mixing hybrids or whatever you want to call it, so a lot of the fan issues with MJ presented are just piled onto everything else.
At this rate, if changes are really going to bother you, better off just sticking to comics or cartoons you enjoy if you don't feel like waiting for the film to either ease your mind or cement your fears.