Premium to Upper upgrade one way

Hi guys,
I wondering if it was possible to buy a return flight to Orlando with one way premium and one way economy, and then use 20k miles to upgrade the premium flight to upper straight away? Are there any restrictions with this set-up apart from the obligitary £100 upgrade fee?
P.S. For anyone who might remember I was going to Tokyo with Virgin, the flight was exceptional!
I wondering if it was possible to buy a return flight to Orlando with one way premium and one way economy, and then use 20k miles to upgrade the premium flight to upper straight away? Are there any restrictions with this set-up apart from the obligitary £100 upgrade fee?
P.S. For anyone who might remember I was going to Tokyo with Virgin, the flight was exceptional!
