You receive the eVvouchers on your statement report ( electronically if you access the web and dated of course) .They also show on the top right corner next to the milage summary.
I have so far received 3 and they work fine, if you wish to upgrade, ask at the gate, and the duty manager CAN see the number of eVouchers you possess, if you do not have enough, and still upgrade, the corresponding milage will be deducted!!