Should be fine as long as you run the install of Windows 7 Professional from within Windows 7 Home Premium and choose the upgrade option. However personally I would never consider doing anything of the sort without making sure I had a strong fall back position. Including a confirmed good system image as well as a good backup of all of my important critical data. Windows Anytime Upgrade: Windows 7 Professional for Windows 7 | Windows 7
As Trouble states vreate a good image before doing the upgrade. If you have a problem with the Anytime Upgrade, restore the Image, then install the Win7 Pro directly into the Win 7 HP partition without choosing to format (this chore will be handled by the installer) Doing the Custom install in this manner should create a folder called windows.old in the root directory. This folder contains the entire old OS. You can open this folder and drag all data to the new OS. Once you have everything moved you wish to save, delete this folder as it is very large.