if you are planning to return within a year from the purchase and usage of the power pass, then it would be worth it.
power pass is valid for park admissions from the first day you use it (not buy it) for one year.
there are black out dates for the power pass.
check the UO website to see which dates the pp is not valid for admission.
sorry, reread your question and forgot to really answer it.
i would call guest relations and ask them if there would be a charge to upgrade those tickets you bought.
or, you can send an email to them to get the current policy on upgrading.
if you include your phone number, it is possible a TM will call you