WOW I guess I was wrong....... See the link
http://www.cic.gc.ca/english/resources/manuals/bulletins/2012/ob423.asp
from what I read, if the only thing holding up the officer is that the medical is expired or about to be expired (because you cannot enter Canada with an expired medical) then he can ask for an extension for a medical officer to look at the medical again. Not because they haven't made their decision yet. They would have already made their decision but cannot get the visa because of an expired medical then they would ask for the extension, if they are still in the process of making their decision and your medical becomes expired then yes you will have to redo it.
As for being "proactive" as i stated, I don't feel that is wrong because at least you don't have to wait for the officer to let you know then go get the medical done, then send it back in etc......they will already have it in their hands. It cannot be screwed up because the medicals all have dates on them and they would use the most recent. I know a girl that has done this and it has not affected hers at all.
Before you belittle people for their "opinions" you should reword your message. I was only trying to help and he does not have to follow my advice, that is what the forum is for to help others not to belittle what they have to say. Sorry I won't be helping anyone in the near future.