If you are Lifemiles Elite:
1. You must book on economy limited seats or all seats rewards tickets.
2. Check how many ecoupons you have and call the call center or request an upgrade with ecoupons online. (You can also buy them) the upgrades starts 24 hours before departure and you most not do preckeck in (TACA told me that)
Or if you booked all seats rewards ticket and have elite status you might get it.
All this based on experiences on TACA.