Can a returning NSMen reapply to another University on the year he is matriculating?

For context, I have a placement in NTU however after much thought I want to apply to NUS instead. But I had alr accepted the enrollment to NTU earlier in January, scheduled my medical appt and I saw on the website that you must matriculate between 12 Mar and 15 Mar. Does that mean I cannot reapply elsewhere since other universities application will only get back to me after 15 Mar? Hope to gain some clarity as I do not want to lose my placing 🥲