“Purdue, UIUC, and UCSD should be considered reaches for the CS major. Getting admission to the school but not in the CS major is more likely, but changing into the CS major after enrolling requires another admission process that should be considered a reach.”
Specific to UIUC, transfers into CS are not even allowed. They do have hybrid CS + X majors which have higher admission rates, but those would also be in the reach category. They do seem to like full pay internationals if you fall into that that category it might help you.