I applied to life sciences as first priority, and chemical biology as second. The requirements for the honours degree of life sciences this year were changed, and for numerous resulting reasons I want out.

Is it possible, having sufficient grades and courses, to switch from life sciences to chemical biology after having finished first year?

You might be able to shadow the program. Take the time to go see an academic advisors.

The school year hasn't started, you can most likely still switch in. Just contact the faculty office

Keep in mind chembio has limited enrollment and many of the classes have a hard cap as to the number of students (for lab reasons, usually). Just having the published average is not enough. You have to meet the actual cutoff AND there needs to be space in the program (due to people who were accepted into chembio going in to other programs).
It's definitely worth a shot and you should contact them ASAP and it might be fine, but it's not guaranteed.