Purchasing tickets - can I upgrade?

If I buy a single day ticket, can I change to a multi-day ticket, if I convince DH to do a second (or third?) day? Will they just charge the difference in price or will they only give me a credit for the value day (when I’m buying a regular day). Or will I just have to buy the multi-day pass and save the single day pass of another day?

Also, if the date I’m going changes from a regular ticket day to a peak, can I just upgrade my ticket?

will you know if you need extra days before you use it? You cannot upgrade unless you have says on your ticket so once you enter a park you would lose all value?

Yes, I’d know before the trip, but that’s a good point. Once a ticket is used, it wouldn’t be upgradable.