Originally Posted by
Flying for Fun
There isn't a mechanism to upgrade a revenue non-Saver economy ticket to Business with miles on AS ...
it is certainly possible to
book a flight using miles as the upgrade mechanism when U is available

the confirmation email will show you as confirmed in Main Cabin (M), and you’ll get a “You’ve been upgraded” email a few minutes later
what I don’t know is whether you can call in to upfare an existing ticket *and* apply miles, although it’s indeed possible to apply a GGU in conjunction with an upfare