In nearly ever case you can upgrade your tickets anytime within 14 days of the first day you use it. There are some exceptions, for instance. Maybe you purchased your tickets from a seller on Ebay and the ticket has already been partially used, that ticket would technically be not valid as tickets are Non-Transferable. This ticket could not be upgraded even if it were within 14 days of its first use. Or, you've purchased a special promotional ticket that cannot be upgraded. (conventions, groups sales, etc.) I suspect that you booked your stay at a Disney hotel, using the dining plan and that your Room key will serve as your ticket. In this case you wont have any problems at all.
One word of caution about upgrades. Disney increased its
ticket prices in 2006. Many "authorized" ticket sellers purchased a large bulk order of tickets from Disney at the end of 2005 (at the old rates) and are now selling those tickets to guests. (at the new rates) Just drive up and down highway 192. You might run into a snag in terms of what you have to pay when you go to upgrade those tickets.