Upgradable is subject to availability. SQ have two upgrade options: through your Krisflyer account you can pay a fixed amount of points to upgrade well in advance, or through mySQupgrade you can bid cash or points to be considered for an upgrade. Both are subject to availability: if the higher cabin class is heavily booked, or for any other reason (e.g. they simply don't want people to get in the habit of flying PE cheaply), then it won't be offered. Particularly with mySQupgrade it's not obvious what pattern of allowing or accepting bids they follow. (With the fixed-price method you can usually upgrade if they are offering award tickets at the time of booking.)