I think that to access Extra, ( as in prior to clearing, I think sometime around May), a student has to be holding no offers to be eligible. If I remember rightly UCAS then publish lists of any courses that have available places. At this point a student can then apply but I am not sure if you might then need a differently angled PS.
I would advise caution around withdrawing from a course- make sure that you are not withdrawing from UCAS altogether as this will cause havoc on clearing day as you will not be able to get a clearing number if you have withdrawn from that UCAS cycle. In the bun-fight that is clearing, I am sure that if a candidate is on a waiting list for medicine, having a place for something else through Extra won't affect that. I guess UCAS would be able to answer. I have found them helpful if I have ever rung them.