Parallels 4 trial to 5.

Hello. I decided to try Parallels so I can use XNA / visual studio without having to restart and load Boot Camp. So I decided to try the 14 day trial or Parallels 4. I really liked it, besides processing slowing down when I'd start up or shut down the VM ((but thats understandable, launching adobe does the same)) same with it shortening my battery life. But I digress.

My question is this, I was running Parallels 4 and my 14 day trial ran out so I went to go buy the full version from my school store, but they were out of stock on 4 as they will be getting 5 this week. Now I read that if I bought 4 recently i can upgrade to 5 for free, and as I had the trial all I really need is to buy the activation key. Now when I buy 5 from my schools store, can I use it's key to activate my trial and then just upgrade to 5? Or if I try to install 5 will it see that i have 4 and just be like "Hey want me to just upgrade that instead of doing a full install?"

Thanks,
~Glitch