If you have a flight booked on day X (i.e. Thursday) leaving from city Y at 23:59 and you want to upgrade:
Prestige: Call in X-2 days (i.e. Tuesday) prior at 00h:00m:01s in the time zone of city Y
Elite: Call in X-4 days (i.e. Sunday) prior at 00h:00m:01s in the time zone of city Y
Super Elite: Call in X-7 days (i.e. Thursday) prior at 00h:00m:01s in the time zone of the city Y