Be sure to save externally a Win7 backup image so you never have to reinstall again.
If you do, then boot the Win7 Pro DVD to reinstall.
Full retail Pro can also migrate to any other machine of your choice for life, as long as it's on only one at a time.
You can reinstall Home Premium to the current machine also at any time if its the native OS whose value is not being realized while Pro is on there.
If HP is also a retail copy, then it too can migrate to any machine of your choice.