How can I reinstall an upgraded copy of Parallels?

So here's my issue: I have a Macbook Pro that had Parallels Desktop 5 installed. I then upgraded to Parallels Desktop 6, and then to 7, all through downloads. Now, I have a new laptop and have purchased a new copy of Parallels Desktop 8. So that new machine will be fine.

But I want to wipe clean my old laptop and give it to my daughter... and reinstall Parallels Desktop 5, or 6, or whatever, and at least get her up to version 7. (I may later upgrade to version 8).

The question is: How do I do this, without having to "buy" the updates all over again?? What installer can I download and use to give me Parallels Desktop 7 right away (and use the existing license, since even after cleaning, it will be on the same machine)?