First of all, that advice applies only to tickets bought at some sort of legitimate discount price. If the tickets were just usual tickets bought through Disney, then there's no advantage to waiting to upgrade.
But now to your answer ...
I'll make up dollar amounts here, just to make it easy to understand.
Let's say you buy a 7-day ticket from an authorized reseller for $200. Let's say Disney sells that same ticket for $225. Now, let's say you want to add a day to that ticket, to make it an 8-day ticket. The only way to add a day is to do it through Disney itself. So let's say Disney's price for an 8-day ticket is $228.
So you take your unused discount ticket to Disney to add a day. Disney will know (from the info accessed via the magnetic strip on the back of the ticket) that you paid $200 for that ticket. And they know that they charge $228 for an 8-day ticket ... so to add that 8th day they will charge you $28.
HOWEVER, for some reason unknown to mortal men and women, once a discounted ticket is used, Disney is unable to find out that you bought it at a discount. So after you use the ticket at least once, THEN go to a Disney ticket booth to add that 8th day. When they swipe the ticket, it will look to them as if you paid $225 for the ticket (the Disney price) rather than the discounted price you really paid. You will be charged $3 to add that 8th day.
Make sense???
