I would highly doubt they would upgrade in Hawaii to a convertible for Avis First members. Even at Avis in the "other 48 states", they are hard to get as First members on Intermediate/Full Size reservations. I'm sure nearly everyone would want one out there.
The standard advice:
-Book the lowest price car you need or are willing to accept
-Ask for an upgrade and hope for the best
If you really are set on a convertible, your only guarantee is to book one.