Can I upgrade to purchase the movie

There isn't currently any way to upgrade a rental to a full purchase, you will need to pay the full price for purchasing it.

But though all purchases are considered final, you could try contacting iTunes support and see if they will refund or credit you for the rental : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption