I asked this question for an international flight LHR to MIA.
I was told yesterday that you can upgrade an Award Ticket to another Award ticket of a higher class.
Eg: Coach award can be upgraded before you fly to a Business Award. This is to be done at reservations.

But you are right that you cant do an EVIP/(Paid ticket with Miles) upgrade / sticker etc. It's just a different type of miles upgrade... DOH!! Confusing