Upgrading a 4 day expiring pass to a 5 day expiring pass costs nine dollars if it is less than 14 days after first usage.
Currently they pro-rate the gate price of a (pre-2005) hopper pass as of the date the pass was manufactured when you do a trade up to (post-2004) Magic Your Way passes. But ask what the trade up value is before giving the go-ahead to the CM to do it. A different less favorable formula is used when trading towards an annual pass.
For hopper plus passes, whether or not any plusses are still there does not contribute to computing the trade up value but if zero days and some plusses were used, something is deducted to arrive at the trade up value. Unused plusses are not added to the water park fun admissions you are purchasing with the new pass.
Should you trade in a 7 day hopper plus pass one of whose days was used, and get a 10 day pass? I am guessing that trading in those old passes is a tossup, bargain wise.
A non-expiring pass is best purchased when you foresee sometime in the future another vacation short enough that the remains of the pass fully cover it.
(Caution: Math Ph.D. required to read beyond this point)
You paid $320. apiece for a 7 day hopper plus and you used one day so 6/7'th of the price is $274. for trading up with. You need to add $141. to get a 10 day non-expiring hopping pass with water park fun which gives you four more days, one more water park admission, and ability to choose Disney Quest which you did not previously have. I value the 4 more days at $31. each and the one more water park admission at $16.
If instead you bought the 10 day non-expiring for $415. without trade in, I value its contents as 10 days at $33. each and 5 water park admissions at $17. each. The old pass still in your hands with its $274 trade up value has six more days at $34. each and four plusses at $17. each.
(Roughly I valued each remaining park day as twice the value of each remaining plus.)
Disney hints:
http://members.aol.com/ajaynejr/dispass.htm