However, unless they've changed their policy, the AP will start on the date the hopper pass was first used, not the date it was upgraded, so you really don't gain any days doing this.
I think the only advantage would be that you could possibly get in to DL on blockout days and then upgrade to an AP with block out dates (such as the Deluxe). For example buy a hopper, go Sat and Sun then upgrade to a Deluxe that has Sat and Sun blocked out in Summer (I think it's still blocked out then). However I'm not sure if this would work, they could make you only upgrade to a Premium since it was used on block out days (don't know if they do this, but it wouldn't suprise me).
If you haven't bought your hoppers yet (or even if you have) I suggest you give them a call and check to make sure.
Also you will be credited the price paid for the hoppers. If you got them on some special at a good price, (say the pay for 3 get a 5 day deal I know they have previously had) then you only get a credit for the price paid (the price of a 3 day hopper, not a 5 day hopper). So really if you know you want an AP there really isn't much of an advantage (except if the above example about the block out dates is true).
Also just because the policy is one way today doesn't mean it will be that way next month or even tomorrow. A few years back we talked to them about upgrading our passes, and were told the current policy. A month later when we decided to upgrade they had changed the policy.
Also I know when they had the Magic Kingdom Club years ago, my aunt bought my cousin an AP (this when when they only had 2 types). He got a MKC discount. A few months later he decided to upgrade, and he no longer got the discount MKC price. It was kind of weird. It was something like regular price was 99 for regular and say 149 for Premium (don't remember the exact prices) with MKC discout it was 89 and 139. He paid 89 for his AP, and when he upgraded he had to pay 60 instead of 50.