Thanks! I agree that my phrase “virtually impossible” is probably too strong, as there is a pathway. It is “unlikely,” according to UW’s official guidance. However, at the admissions event we attended at UW, we heard stronger language than “unlikely.” They actively discourage students from enrolling in UW with the plan to declare a CS major at a later time.
From Admission to majors | Office of Admissions
“If you’re not admitted directly as a freshman, it’s unlikely you’ll be admitted to either computer science or computer engineering as a UW student.”
From High School Students | Paul G. Allen School of Computer Science & Engineering
“if you are absolutely certain that you want to major in Computer Science or Computer Engineering and do not receive a DTM offer, you should strongly consider attending another university.”