Except that if you order the extra days ahead of time through a ticket broker, you might do better. If you wait, you'll pay Disney prices to upgrade. So the prices you pay probably won't be the same. Close, but not the same.
When I compared my family's tickets, I found it better to buy the extra day ahead of time, but when I compared buying 6 vs. 10 day no extras tickets thorugh UCT, then the difference actually worked better to buy the extra days AFTER you were at Disney. IF you upgrade after you use them at least once, you'll only pay the difference in Disney prices.
Comparing 6 day tickets to ten day tickets (no extras) I get
UCT - 6day to ten day =$12
Disney - 6day to ten day= $9, but their overall prices are higher.
If you bought your 6 day tickets through UCT to Disney - you'd pay the UCT price, plus $9. So that would be your best deal in this case. Though the savings is only $3 per ticket.
On the other hand, October 2007 is a ways off. You could certainly wait until your trip was closer to buy your tickets, in which case you'd have a better idea of how long you intend to go. Disney will probaly raise prices between now and then, but it probably won't be that much. There's also always the chance that Disney will offer a discount for your travel dates.