If you don't change your booking, UA will deny you check-in and boarding at YVR. I assume you're flying on a day when the direct YVR-IST flight doesn't operate. Another alternative to what TK proposed, if you have date flexibility, is to change to a day when that flight does operate. A third alternative would be to find available UA award space from YVR to any TK gateway on your desired date of travel and then continue on from there on TK.