Note - for upgrading Park Hoppers purchased with Airmiles - use the tickets first (i.e. visit your first Park) and THEN upgrade the tickets (adding days, adding hopper etc.). This way you will receive the face value of the ticket and only be paying the difference between face value and whatever your upgrade is (i.e. if you buy a 3-day Base ticket with airmiles and want to add an extra day at the Parks, you will only pay the difference between the face value of a 3 day and a 4 day). If you upgrade first, without using, then Disney knows what 'discounted' rate Airmiles bought the pass at, and you end up having to pay the difference between the discounted rate and whatever you are upgrading to.
We have always bought the 3 day base and then used them once, and upgraded to whatever length we needed and added hoppers at the Park, with no issues.