Now EWS is a separate reservation. If lets the cut off for a particular college is 450 marks for both EWS Reservation and General Category. You submitted EWS certificate. Now you secured 450 but the seats under EWS Category are filled but General Category seats are vacant. You will not be considered under EWS Category. Candidates belonging to General Category who have scored less than 450  will be given chance. The cut off for General Category will be reduced and the Candidates belonging to this Category will be given a chance. You cannot swap your Categories because there are seats vacant. That is not possible. Once you submit your EWS Certificate, you will be considered  under EWS Category only.