You always do the initial medical voluntarily! When you file your initial application for the first time, there is a request for medical form included in the package. Fill that out and take it to the doctor. It includes questions like file#(normally people leave that blank since they havn't applied, in your case you would have that by that time and can include that as well), it also asks where your case is going to be processing, you want to write Islamabad Pakistan in there and based on your DOB first and last name they will attach it to your file. This way you have very little chance of getting the remedical request. And yes you are right, you need the old expired certificate and the request letter from the embassy to take it to the doctor when going for remedical. HOpe this clears some things for you.