My AAA Agent was an idiot and told me to decrease by one day I'd have to PAY $100 more for 2 tickets. I know in actuality this would be a couple dollars.
Tickets before the new Magic Your Way tickets became available will have the option to upgrade to a new ticket. Park Guests can generally upgrade to any type of ticket by simply paying the difference, provided the new ticket is of the same or higher value.
You can upgrade most any ticket, but once you purchase, you can't downgrade. If you've already purchased your tix and fear you may not use all the days, the only way to get your money's worth is to add a no-expire option to them.