There is a deadline for making changes to packages. Before that deadline, you can modify tickets to have more or less days and options and that does not count as your one add-on. All tickets for everybody in your party have to be modified to end up the same way.
Between the package deadline and check in I'm not sure whether tickets can be modified or how.
After check-in and the tickets are handed to you you can add but not delete options, each ticket can be added to differently, and I'm told each ticket has just one chance to be so upgraded*
*Have I been wrong for weeks? I was told that "upgrade" meant adding days in the sense of sixth day costing $3. since that is the difference in posted price between an all new 6 day pass and an all new 5 day pass; adding hopping costs $37. which is the same price posted at the ticket booth. I have used the term "trade-up" for applying the dollar value, prorated according to partial use, towards the purchase price of another ticket.