When I book my Exec Aisle selection, I first book mid-size and check the price, then I switch via the car selection to full size and make sure it's the same price (sometimes the website is quirky and it's not the same, but most of the time it is). If the price is the same, then I select that. When you get your confirmation the top part will now say Executive Selection (instead of Emerald Aisle Selection). BTW, I would only change if it doesn't effect your price. Did they send you a new membership card? I always carry ours and give it to the rep at the Kiosk just so they know that we are Exec Aisle, even though my account shows it too.
Congratulations on your upgrade. Did you upgrade via alot of rental days or did you use the Am.X perk? I'm old school and have always gotten my upgrades by renting 40+ days a year....