Well, we did this about 2 years ago, so I'm not sure if things have changed. We weren't going to a "lower" pass, just changing a 5 day for another 5 day.
You only get what you PAID as your trade in value. So if on Monday you bought a 5 day pass for say $250.00 (I'm making up these numbers for demonstration purposes) and used one day of your 5 on Monday - then on Tues. went to "trade in" the 4 remaining days, that ticket would be "worth" $200.00 (as prices wouldn't increase in one day's time). A "new" 5 day pass would cost you $250.00 (which is what you paid on Monday) and you have a $200 trade in, so you would have to pay $50 more to "upgrade" that pass to a new 5 day.
Again, this was some time ago that we did this, and I don't know that it saved us any money. Hope this helps.