If i understand your question, all you would need to do as an MVPG to get an upgradable at booking fare is click on the MVPG upgrade button when you are selecting the flight. That should filter so it displays the fare that is available for the instant upgrade. Be certain, however, that there is upgrade space available as shown by the solid blue F box. If the box is not solid blue there is no U space and you will not get the instant upgrade.

If you are trying to upgrade to PE then when you hit the MVPG filter there should be a column displayed that says Premimum Class Upgrade and you simply select the displayed fare. I have no idea how you can tell if there are PC upgrade space available.