Today's Q can come in many forms: "Do I have to do it this way?" "How much freedom is there to change things?" "Can I still use my favorite activities?"
Just like a dish you want to eat is a cohesive whole, people need to think of a curriculum as a coherent, connected, fairly complicated whole, with dependencies. Standards are one thing—they are a statement of what kids should know at the end. A curriculum makes choices, and choices have consequences. We set up pins in October that we knock down in February. After students have a well-designed opportunity to learn a term, idea, or skill in one unit, we believe that they will be able to remember it in a later unit. This is what you want out of a curriculum. You want kids to be able to make connections between ideas.
The starkest example of this is a question we got from one of our pilot schools: "The word slope just shows up in grade 8, unit 3, as if the kids are already supposed to know what it means. This is terrible! What is going on here?" What was going on was, they skipped units 1 and 2, which were about transformations, thinking transformations were less important, and jumped right to the unit called "linear relations." The end of unit 2 takes a transformational approach to understanding the meaning of slope. (We use dilations to understand what it means for polygons to be similar, learn properties of similar figures, and then use slope triangles (similar right triangles with their hypotenuses lying on the same line) to show why we are allowed to refer to the slope of a line.)
Just like a new recipe, you kind of have to teach a coherent curriculum the way it is written for a couple years before you really understand what is in there. Then, you are in a position to understand what it is safe to substitute or rearrange.