Easiest way to upgrade bulk of machines from Windows 7 Pro to Windows 7 Ultimate or Enterprise?

Looking into full disk encryption and rather than purchasing something like Kaspersky or Sophos encryption for my Windows 7 Pro machines, it seems easier to just upgrade the machines from Pro to Ultimate/Enterprise over time.

With that said, Windows Anytime Upgrade is no longer a thing. What's the easiest way to upgrade? I'm noticing Microsoft is making it really hard to purchase any Windows 7 keys or maybe I'm just blind.

So my two questions are:

1. Where to purchase bulk Windows 7 upgrade keys?

2. How to deploy without too much user inconvenience?