I'll be happy to answer...
Yes, you can upgrade your ticket anytime before it expires (14 days from first use) by adding additional days or any of the options. If you add the "no expiration option" the price of this option is based on the original number of days on the ticket, not the number left.
Also, dave2822 is correct that you can only use one theme park day per calendar date. You can however use multiple magic plus visits (valid for admission to:Typhoon Lagoon, Blizzard Beach, DisneyQuest, Pleasure Island, or Wide World Of Sports) on the same date if you have added the Magic Plus Pack option to your ticket.
Hope this helps! :sohappy: