If it is an unused pass, then you should be able to upgrade by paying the difference in price. However, they may make you upgrade to a PAP since your ticket has pluses on it. And the upgrade cost will be based on what Disney got for the ticket. If you purchased it from a discount ticketbroker, they will only apply the amount that the ticketbroker paid for it towards your new pass.
There are other rules that would apply to partially-used passes.