I think that despite the change movie does a good job respecting the book. Why? Because first of all having a 12 year old have a “weird” dream about a girl is kinda creepy, it's normal but it's creepy. Having him be older in the movie is just a small change, it's not going to make any big changes in the story, plus in the movie they have kissing scenes and watching kids kiss can and may be weird for some people (like me).
Another reason why the movie still respects the book, despite the changes is that having Asher as a drone pilot instead of a teacher would create more drama. The movie would be boring if there was no drama and having friends show trust, when having hard decisions is interesting. Like we have the romance between Fiona and Jonas but i think that we also need to include a friendship not just romance because not everyone is comfortable with kissing scenes or stuff like that. I'm fine with it but I know people that are not comfortable with and turn away when things like that come up. And when friends are put on the line it's intriguing to see what they are going to do or say.
I think that the movie does a good job respecting the book despite the changes. There is one big difference that affects the story is Fiona being a nurturer to a nursery worker. By being a nursery worker she helps Jonas escape and then getting in trouble and then plan to release her. She help Jonas by getting another basket the same one the Gabriel was in and runs with it tricking the guards by thinking that she has Gabriel when really Jonas is running the opposite way with Gabriel.
Another reason why the movie does a good job respecting the book is that they still keep the characters the same and the meaning of the story too. By that I mean that they don't change the book completely they make small changes to make it better, like Asher and Fiona's job and also that they are older. They did make the Chief Elder a bigger character than in the book, making her appear more and making well evil. The story is still the same, people are afraid of difference, standing out from crowd, today calling someone different is more of a compliment than an insult.
The final reason why the movie does a good job respecting book is that having flashes of color and having like a birthmark instead of their eyes being a different color isn't really a big deal. First of all the flashes of color and the apple changing color isn't really a big deal, they still got the message, it's still showing that he is seeing color. Now the eyes and the spot on his wrist isn't much of a difference it's just that they are in different place, the important thing is that he can still receive the memories. And that is why the movie does a good job respecting the book