1) Consider shrinking HP, then creating and formatting a new partition for Pro using its installer. This will create for you a temporary test partition for your purposes.
2) Normally when you upgrade between Windows 7 versions you must use Anytime Upgrade, however you can adapt a workaround to do an in-place Upgrade and later even use it to downgrade from Pro to HP.
To do this, adapt this workaround by precisely changing only the two registry keys shown, which make the installer think it is doing an (always allowed) Repair Install: Upgrade the Windows 7 RC to any retail version « Icrontic Tech
Activation will be reset so you can use the new Product Key.