I only have Canada work permit right now so can I file the transfter application after I get Canada green card or I should file it before I leave US?
-You had better file the I-824 now or shortly after you settle down in Canada and indicate the interview in China, not Canada. But remember that you need to find a US employer to offer you a permanent job before you are interviewed for the immigration visa when you PD becomes current.
I have ted my medical examination along with 485. Will I need to do another medical exam again in the future?
-Not need to do the physical exam again if you file the I-824 before your I-485 is denied.