Just had a look at cost and 7 days is border line on a 10 day ticket. Doesn't matter if you use 1 day or 9 days its still costs $226 to upgrade to non expiry for the remaining days.
If you used 7 days then pay $226 the remaining 3 days cost you $75 each to keep for a future trip. possibly still worth doing if you only want 3 days for another trip, if you want more than 3 days on next trip probably worth buying a new ticket.
You need to look at costs and how you use tickets to see if its worth it, its more expensive to do than I expected.
Details & costs here
http://www.mousesavers.com/wdwticketprices.html