I don't mean to add fuel to the fire, but some DMP's refuse to perform medicals if applicant doesn't submit a request from the VO! I've seen this happen to someone.
So, it depends on the VO, the specific DMP and probably your specific case. There's a 50/50 chance that you'll be asked to re-do the medical, hopefully not, but if they do, it's probably wise to plan carefully, maybe ask your DMP first if they would re-do a medical without a VO's formal request, and why not, call the VO and ask them if a medical re-examination would be required in your case.
I know of one forum member, alex121 (http://www.canadavisa.com/canada-immigration-discussion-board/thread-for-people-using-the-guatemala-office-t44160.0.html;msg919015#msg919015) whose medical was about to expire in about a week, she did not jump to re-do the med, instead waited for the VO to communicate with her, they finally contacted her to see if she was ready to fly and land within 5 days or so, she responded she was not ready on such a short notice (i don't blame her!), so they told her "in that case you'll have to redo your medical and wait for the visa".
So take your chances, but plan as much as possible.