The results of a medical examination are sent by the DMP to the Regional Medical Centre that represents the CIC office or embassy that will be processing the application. Inland applicant results are sent to the RMC in Ottawa- and that office also serves some of the outside embassies, like the US. So it may be that it won't be an issue at all - depending on which overseas embassy will process your outland application. I'm assuming, from your avatar, that you're probably from the States, so if Buffalo will be processing your application, you'll be okay. Buffalo requests medical results from the RMC in Ottawa. This was exactly the same situation I was in and, although Buffalo seemed to have difficulty obtaining results for one half of my family members, but not the other, you might include a note with your medical receipt letting them know that you had initially intended to apply inland. That's because (unless they've changed things in the past 3 years) the DMP will have only given you a receipt showing you paid for the exam, when an outside Canada DMP will actually sign a form that you submit with the application.
If, for some reason, Buffalo notifies you that they don't have your medical results, all you have to do is fax the RMC in Ottawa with your information (name, DOB, when and where you were examined) and they're really good about tracking them down and forwarding them on to Buffalo.
