the wpc circulars mention that it is necesary to have a valid cpl and frtol. there is no mention of "current" cpl/frtol or otherwise. your canadian cpl is valid for lifetime. your cpl does not seize to be valid just because your medical is expired. you cannot exercise the privileges of your licence of course, with out a valid medical, but your cpl remains valid.
furthermore, in order to convert/renew your rtr permit, there is no requirement to provide a copy of your medical.
example: the wpc circulars mention that "' Issue of RTR (Conversion) and RTR (Permit) certificates and their renewals shall be for a period of 20 years or till the validity of FRTOL licence/ CPL whichever is earlier." Previously, the wpc has been granting a 3 year permit based on a converted rtr. The class 1 medical of most (if not all) countries is valid for a maximum of 1 year. Therefore, if it were a requirement to have a valid medical in order for your cpl to remain valid, then the WPC could not issue a permit for a period of 3 years, let alone 20/lifetime. All permits would be restricted to the date of expiry of the medical, instead of the cpl.
In short, there is no need to keep renewing your foreign medical.