I am looking for some clarification on when upgrades clear if they are 'purchased' with miles.

If I indicate when I purchase my ticket that I want to upgrade with miles, does my upgrade request go in the same queue as elite members upgrades (where as a 2P I have to wait until 48 hours before the flight for them to clear)? Or do upgrades purchased with miles clear right away if there is a seat available?

The only rule I can find on united.com says that the "ticket must be issued in paid class before an upgrade can be issued." But it doesn't say when the upgrade will be issued - 10 minutes later, 10 days later or maybe 10 minutes before the flight??

Here is my situation. I will be flying IAD-NRT, and have enough miles for a standard award for business class. But if I can buy an economy class ticket and use miles to upgrade, I'd rather do that. But only if I can know for sure when I buy the ticket that I can get the upgrade.

Thanks for any info on this!