When you register for courses during SO, you will not have an immediate opportunity to declare your major. However, you can take care of this very early on in your first semester, if you choose. You’ll have to fill out a form with your academic advisor and submit it.
HOWEVER, I want to clarify that there are not ‘undeclared engineering’ courses. All first year engineering students are taking most of the same courses anyway. A student who has declared ChemE will likely take the same courses as an undeclared student who is considering ChemE. (I say likely because the courses you take are up to you.) I hope this makes sense.