They do sell the Premier pass.
I am assuming you bought direct from Disney, so they will probably know how much you paid and use that as the amount towards a Premier upgrade. If you had Platinum, I assuming $780 cost; Premier costs $1580, so a $800 extra cost. That is cheaper than a Deluxe pass alone. A Premier will allow for no blackout days, free parking, better discounts, and included Maxpass.
Considering a park hopper is $167, parking is $20, if you will definitely be visiting more than 4-5 times it will make sense. But an AP will help you visit in a more relaxed manner-going for a couple hours after work, stopping by for a casual weekend afternoon, etc.
Best way to upgrade is at guest services at Disneyland, just know there could be issues connecting the two systems so you may have some difficulties, but they usually try to resolve ASAP and make it right.