You don't pay extra to upgrade at WDW versus purchasing your entire ticket in advance. For both your questions, there is a ticket sticky at the top of this forum called "Everything About WDW Tickets." Upgrading tickets is discussed in Post #6 "Upgrading MYW Tickets (Not to Annual Passes)" and current June 3, 2013
ticket prices are found in Post #18.
The ticket chart in Post #18 is a great source of info. Be aware that the prices posted are pre-tax. Our group has purchased less days than we needed when we made our reservations. Ticket costs can add up, and it's especially difficult to dig up all the money if you are getting a package and need to pay for your package in advance. So adding days once you get to the parks gives you more time to save up. If you are purchasing a dining plan, the requirement is purchase of a one-day ticket for each person in a room. For Free Dining, it's required to purchase a two-day ticket for each person. Any more advance ticket purchasing is optional on your part.
If you study the chart, you'll see that Day 1 base ticket is $90 or $95, Day 2 is $184, Day 3 is $262, Day 4 is $279, Day 5 through Day 10 cost $10 more incrementally. So the expensive days to upgrade to are Day 1 to 2, Day 2 to 3, and Day 3 to 4. After Day 4, adding another day is just $10/day.
As noted above, Post #6 explains upgrading at WDW. It may be too technical for you to understand if you're a novice. Basically, if you bought your ticket from Will Call or a Disney Ticket Booth and there's a price increase, you have to pay the price increase in addition to the upgrade you want to do. To avoid that, most people get their tickets with their Disney resort reservation, or from a
Disney Store, or from a 3rd party Authorized Agent who offers a discount (such as
Undercover Tourist who everyone here highly recommends). These tickets will not be charged the extra after a price increase AS LONG AS the ticket is used first at a turnstile to enter a Disney Park. After using the ticket, the value of the ticket is elevated to current gate price. In that case you pay for your upgrade and you get to keep your discount, if any. However, if you do not use your ticket first before you do your upgrade, you will owe any price increase. So this is important info that could save you quite a few dollars. If the Ticketing CM does not give you current gate price credit for your ticket and you followed the instructions above, take back your tickets and find another Agent to deal with. It sometimes happens.
Tickets expire 14 days from first use. You don't have to use them consecutively. If you are staying at a Disney resort and have a KTTW card, you can upgrade anytime during the 14 days even if you've used all your ticket assets, because your card has your name and you can show ID to prove the used tickets are yours. But if you are staying offsite and have tickets that need to be upgraded, you must do the upgrade before the last day on your tickets is completely used up, even if the 14 days is not up. The issue here is about ticket scammers, and it's another story.
Here's a link to the Ticket Sticky:
http://www.disboards.com/showthread.php?t=1452684
