I've been trying to understand a nuance about the upgrade feature without success: if I book an award ticket and there is already award space available for the next cabin at the time of booking, is it possible to immediately upgrade with miles to the next cabin? Or is still only within the 30 day-72 hour window (and hope that the space remains)?

I ask because the total miles required via the aforementioned method would be lower than booking straight into the desired cabin...