This is a slightly complicated question. The DIS used to have a sticky on the subject, but sadly the fabulous CM who oversaw it passed away. He helped us so much!
There's another webpage that's very useful, I'm not sure how much I'm supposed to mention other websites. But if you search, you'll find it. Actually, I know two other pages that have ticket price lists.
I'm not sure about parksavers per say, but most 3rd party ticket places pay a wholesale price. (less than the retail price at the gates, and less than they charge you.)
If all goes well, your ticket gets bridged when you ask to modify it, and you only pay the difference between the two retail prices. Seven day hopper retail is $467.54, eight day hopper is $478.19 = $10.49 including all tax.
Other times, the WDW staff says the value of your ticket is the wholesale price. In that case, they will try to charge you the difference between the wholesale value and the 8-day retail price. If that happens, you can appeal to a manager or walk away.
Now, one other thing that is VERY important! If you buy a seven day ticket, and use all seven days, that's it. You can't modify it on (what would be) day 8. You can also ONLY modify a ticket with 14 days of first use. I'm not sure if they let you change it on day 7. Your best bet is to modify it after you use it at least once, but by day 6 at the latest.
