If you can not change your status to work permit by Dec 20, 30 days before your TRV expires, you need to apply to extend your TRV.
Is your employer willing to extend your work permit? For all Foreign workers out there you need to renew your work permit 30 days before expiry date of your work permit but please do not wait till the last minute just in case you encounter problems and you can renew it 2 months before
expiry date. If your work permit has expired you still have 90 days grace period to renew and if you still did not renew your work permit after this grace period CIC will notify you to return to your country of origin.