Using miles does allow you to confirm your upgrade well in advance of the 72-hour period. However, the same restrictions apply in terms of F seats versus A seats. In fact, CO releases inventory from A to F at various times, but they do not disclose it in advance. So you may not be able to get the upgrade when you book, but you may have better success down the road. In any event, I usually just take my chances when the window opens up for most east coast flights, but use miles to confirm an upgrade in advance for trans-con.

By the way, if you're using miles, CO has to issue you an upgrade certificate. And if you're within three weeks (or so) of the date of travel, you'll have to pay an extra fee for expedited delivery and overnight shipping. But the good news is that CO agents are notoriously bad about collecting the certificates...I have one that I have used three times because the agents don't bother to ask for it.