yes you can but you have to pay an upgrade fee on your phone, but its a fee on top of the actual phone upgrade fee.
So say the phone you wanted was £50 for an upgrade, depending on how long you have left and how much you spend with them they will charge another fee, can be £50-£150 though I think.
If you have your account on the Orange website or go to the upgrade section once logged in you can see how much the upgrade fee is for upgrading early as they will tell you online, its normally not a nice figure though Dave. I have always resisted in the past, I have just finished an 18 month contract and it didn't seem that much of a problem really, i had a good phone so didn't really care.