De acordo com o site da Microsoft :
The following OEM versions of Windows software are eligible for downgrade rights.
Windows 8 Pro includes downgrade rights to:
- Windows 7 Professional - Windows Vista Business
Portanto, para responder à sua pergunta, não, você não pode fazer o downgrade diretamente do Windows 8 Home Premium. Se você fosse fazer upgrade do Home Premium para o Pro, voltaria ao Win7 Pro usando as seguintes etapas (também no site da Microsoft):
The downgrade process Follow these steps to downgrade to a previous version of Windows software:
Obtain genuine Windows media and a corresponding product key for the version of Windows that is eligible for downgrade.
- The media should come from a prior legally licensed version from the OEM or retail channels.
- Qualified OEMs have the option to distribute downgrade media and product keys directly to end users, per end user request.
- End users who are licensed separately through Microsoft Volume Licensing (VL) may provide their VL media and key to a system builder to facilitate the downgrade on their own systems.Insert the downgrade-eligible version of Windows media in the CD drive and follow the installation instructions.
Type the product key.
- If the software was previously activated, you will not be able to activate it online. In this case, the appropriate local Activation
Support phone number will be displayed. Call the number and explain
the circumstances. When it is determined that the end user has an
eligible Windows license, the customer service representative will
provide a single-use activation code to activate the software. Please note that Microsoft does not provide a full product key in this
scenario.Activate the software.
Dependendo do seu acordo de licenciamento, você poderá obter uma chave do Windows 7 Pro por muito mais barato do que comprar uma chave do Win 8 Pro. Verifique isso antes de tentar prosseguir. Se você realmente for padronizado, presumo que você tenha um contrato de licença por volume com a Microsoft.