I'm looking at buying a ticket from Undercover Tourist. Will I be able to upgrade it if I need to (for more days) at Disney's window when our trip rolls around (for market price of course)?
Yes, you can, but you should use the ticket first before upgrading it. When the ticket has been used, Disney will "bridge" the price between what UT paid for it and value it at what the "gate price" is.
See the "Everything About WDW Tickets" sticky in the Theme Parks Attractions & Strategies board here on the DIS for complete information.
If you go to the locked sticky Everything About WDW Tickets in the TPA&S Forum, especially Post #6, you can get a lot more information. An easy way to get to the sticky is by clicking on the link in my signature. The ticket sticky is your friend!