You should also take into consideration the chance your flight from LHR to YVR could arrive late. It will be winter so even a small delay of 15 minutes is possible and could make you miss your connection. Your checked bags should be forwarded directly to your Smithers flight so that won't hold you up. You need to decide what is the cost of not arriving in Smithers when you need to be there. Would it be more than the cost of paying to change the ticket and being sure you can arrive when you need to?