Also according to allearsnet.com:
"A second scenario to consider comes when you want to upgrade your old PHP's to Annual Passes (AP's). The upgrade will be handled the same way as above but with two important differences. First, the unused plus options carry no dollar value towards the upgrade. Second, any unused plus options will be lost when upgrading to an AP. Now this may not sound like a bad thing since you'll be getting an AP for a whole year's admissions but what should give you pause to think is that turning in two, three or four unused plus options means that you will have to now pay $33.00 for each water park admission you do. That alone could mean you'd be paying another $66, $99, or $132 for water park admissions during the year that you gave away for free when you upgraded the original ticket."
So, the upgrade procedure is the same as indicated in the previous example - if yours is just a PH, then the extra bit about the "plus" options doesn't apply.