At times I decide I will illustrate a book a certain way, and then when I start doing it, it feels completely off and doesn't come out anything like what I wanted.
This was the case when I started illustrating Billy Can't Slow Down! a book I collaborated on with Dr. Stephanie Margolese.
As you can see, my first idea was to do something with an outline. Once I have an idea, I always do a single page to see how that style would fit in the book and once I did that, I realized I didn't like it. So I went in a completely different direction and went for a style that I felt work better.
All this to say, as the illustrator, you have a better idea of what you are capable of, so don't be afraid to change style if it's not working out for you.