I recommend that people call TK before making any reservations they intend to upgrade using a voucher.

I called TK and my vouchers for the new elite year had not yet been received. The agent suggested that this is a common problem but that Istanbul will issue the vouchers for me within an hour.

Luckily I booked with a US OTA so have enough time to wait before cancelling the whole thing if necessary.

But it would have been less nerve-wracking if I had simply spent 5 minutes ahead of time to call TK to confirm that I have the 2 vouchers I'm expecting to have...