I went and looked up the "official word" because it has been a while since my cousin did his CP and it turns out you're right!

Here is what the hub said:
"Cast Members must be participating in the spring Disney College Program with a May or June 2008 departure date to use this process. (Please be aware that you cannot participate in consecutive programs totaling more than one year.) Spring Advantage Program participants will have an opportunity to apply for an extension mid-summer."
However, they don't say anything that about how that effects applying for PIs. I know a lot of people are applying for CPs as their backup. I would suck if you did a CP but weren't able to extend for a professional internship. Someone should email disneyinterns.com and find out the official policy!!
Here is my advice on a CP to PI transition. Keep in mind that not all information you will read on these forums are accurate and things change time to time.
I would HIGHLY NOT RECOMMEND doing an Advantage program if your goal is to move into a PI.
If the PI program requires you to have completed a CP, then you will NOT be eligible for the PI when it comes to apply. By the time your CP ends, its too early to apply for the next one, but the other PI program is already going.
There is exactly ZERO advantages to doing an advantage program. If I had it to do all over again, I would do the short program and here is why.
1) If your goal is to move into a PI, the short program not only allows you to complete a CP, it also allows you to be eligible for the PI....and you will have enough time to do the PI while you are still down there because it will NOT put you over your one year limit
2) If you get a role or position some where that you dont like, there is very little you can do about changing roles. One of the main advantages of a short program is that you can extend and do something else.
3) I never saw anyone get denied an extension unless they were going to be over the 1 year mark, or had a bad record card.
I came down here with the intention of doing a PI and listened to everyone ( including the recruiter ) and was told the advantage program was the way to go. Unfortunantly, this was totally wrong and if I had done the short program, I would have been completed with my PI long , long ago.