There is a charge to make a change to any package reservation within 15 days of travel. Upgrading avoids the $50 revision fee that would apply if your package change lowered your price anytime within 45 days, but they still have a $10 fee, tomorrow increased to $15 for last minute changes, even upgrades. They've been more strict about not waiving these fees in the last year or so. For package reductions within 15 days you'd get hit with $65 for both fees.
If your change is within 15 days of your trip, just add the PH when you are on site, after using your ticket at least once (to bring it to the current new price). The add on will cost you $54 plus tax, but it avoids the last minute package change fee. If your trip is beyond the 15 day window, add the PH today and it will cost $52 plus tax per ticket.
It's not a charge to make your ticket a PH, it's a charge for any package changes within 15 days. I had a client make an even change on her package a few days before her trip (YC to BC, same price) and she was charged $10 (soon to be $15) for the change.
If you wait until you're at Disney to add your PH, you can do it up to the last day on your ticket, but won't save any $$. PH can be added anytime for a flat rate when you need to have it. If you're on the fence you can wait until you need to hop.
You know, this does sound correct. Two years ago I upgraded the two tickets on our package to water park and more fun tickets at check in, and the price I had to pay was around $20 more than I had calculated it would be.