Have looked into this closely. If you buy a 'super-saver' fare, meaning one of the heavily discounted ones that they offer in their promotional emails, you won't be able to upgrade with points until 24 hours prior to the flight...BUT, if you buy a standard saver, you won't have the 24 hour restriction, but you may well have no ability to upgrade until about 14 days prior to the flight.

Most importantly, there appears to be no advantage to buying flex instead of saver, other than the increased number of tier miles, as the upgrades for flex and saver both come out of the 'D' fare bucket.