Deadline confusion?

Hi So I've sent my medical application the day before yesterday. And the deadline was yesterday. BUT, on track, it says 'If you made a mistake with this choice, you can still substitute it (for a limited period only)'. It's past the deadline for the BMAT too, so IF I DID substitute one of my choices to a BMAT university, will I be wasting an application because I haven't registered my BMAT FOR THAT UNIVERSITY, but another one.

To put it in context, I want to change Lancaster to Brighton and Sussex.
In school, ages before the deadline, I spoke our exams officer and she signed me up to take the BMAT, and the universities I told her to put down for the BMAT included Lancaster and NOT Brighton and Sussex.

So, AM I past the deadline for choosing my BMAT university? or JUST the BMAT REGISTRATION? and if I DID change from Lancaster to Brighton and Sussex, would I be wasting my application because I haven't told my school to sign me up for Brighton and Sussex to send my BMAT results to???? I'm literally soo confused!!

Ask your school if they can actually change Lancaster to BSMS first, because the universities they put down there are the ones who will receive it since if they can't actually change to BSMS then there's no point trying to change it yourself

And I think that if you substitute a medical school in after the deadline the universities will just ignore your application since it was past the deadline, so if you change it in track yourself and then it turns out that's right then you've just ruined your application for both Lancaster and BSMS