Hi Trevor
When you click on the type and price of a ticket (but before you get to the screen to pay), watch on the right side of the screen. There, in "your basket", you can click on the link "View fare breakdown" to see the fare codes. Assuming that you selected the lowest fare in a particular class of service, you will often get a non-upgradeable fare (for example an H fare for Premium Economy). To get an S fare instead in this example, you would have to instead call Virgin.
But, many/most times an upgrade will not be available, even if you have the miles and are ticketed in a valid fare code. There has to be availability in a special upgrade class for the class of service you want. You can get a general idea if upgrades are available by doing a dummy booking using miles and money for the desired class of service, for example upper class. If there are "free" tickets available on a particular date, then often there will also be upgrades available (but not always). You still have to go through the hassle of calling to find out. (I'm actually ok with this as it prevents automated processes from reporting / grabbing all the upgrades for their subscribers!)