An OEM key is now, and always has been, "locked" to the mobo. This does not mean that there is a physical aspect. NOTHING physically happens to your board. It is a digital thing - and the how may not be relevant here. No, an OEM key will not restrict you in any way as it pertains to that particular mobo.
If your OEM key is Home, you can upgrade to Pro.
"I understand that Pro has more features and can be installed multiple times"
Incorrect. Pro refers to a feature set, not the ability to install multiple times.
RETAIL keys can be installed on ONE machine at a time, multiple times.
In theory, there are no future releases of Windows. Windows 10 is a service. You can upgrade the service from Home to Pro to Workstation to Enterprise.
Windows 10 Workstation was once known as Windows 10 Pro, but MS removed/modified four features from Pro.
More info here.
Windows 7 Ultimate/Pro/Business will only upgrade to Windows 10 Home or Pro - not Windows 10 Workstation or Enterprise.
Worthy of note:
There is no functional penalty in running an unactivated copy of Windows 10. There is a watermark and an occasional nag. You can even personalize an unactivated install.
I do not know if PCHelpForum allows discussion of this topic.
Also worthy of note:
Some users still qualify for a free upgrade from Windows 7 to Windows 10. You must claim and warrant that you are a a user of 'assistive technology' to qualify for the free upgrade. Such claim and warrant is performed by clicking an 'agree' button.
This is possible:
Install and activate Windows 7 - using the highest grade license you have (Home, Pro, Ultimate). It does not matter if it is OEM or Retail as long as MS will activate the install.
Online upgrade to Windows 10 (using assistive tech link, if qualified, otherwise pay).
Sign into Windows 10 using the email login. Activate Windows 10.
That PC is now 'locked' to the valid Windows 10 license. The PC Settings / Activation will show "This PC is activated with a digital license".
Wipe that entire install out by performing a clean install of Windows 10. Skip all prompts to enter a license. It will automatically activate.