How can I save my Windows 10 digital licences for multiple installs?

I have this elaborate multi-boot setup with my "daily driver" Windows 10 on my main GPT SSD plus 4 extra Windows 10 partitions on a secondary MBR spinning HDD. I use EasyBCD to multi-boot and it works (or did work) flawlessly until I did something today that was probably really stupid.

I wanted to add some more partitions to that second boot drive because it is 4 TB and there was so much wasted space. So I used MiniTool Partition Wizard (highly recommended) to convert that second drive from MBR to GPT. The free version only converts DATA drives and I didn't want to spend the money for the paid version to get conversion for OS drives. The conversion ran OK.

At this point, EasyBCD throws up a message "Error Opening BCD Registry." I managed to get things back to normal by by re-converting the drive back to MBR. However, if I change the drive letters on those 4 extra partitions to NONE (so Windows doesn't get confused), then I get that error window again. Before today, I had drive letter NONE for those 4 extra partitions, and EasyBCD ran fine.

Now I need to upgrade all these extra Win 10 partitions to 1809 anyway, so I'm thinking that I should just erase all the partitions on the drive and reformat as a GPT drive. Then I would recreate my 4 Windows partitions on the second drive using 1809. These extra partitions are just for scratching around, doing test installs, etc, so I don't have to save anything on those partitions.

Before I do that, I want to be sure that I have copies of the digital licenses so the Microsoft servers activate these partitions, and I don't lose the installs.

The company I used to work for was in some kind of third party developers program and they had 4 extra installs on a Win 7 Retail Edition key and they let me use those installs.

So what are my next steps?
Yeah, I know I did something really stupid.
x509

I have done numerous test installs of just about every Edition of Windows 10 on several of my PC's. They were for the most part all activated with Digital Licenses. I have a My Visual Studio Enterprise Subscription.
I can now do clean installs of those same ditions on those PC's without entering a key and they auto activate with the Digital License they got when first activated. I just skip entering a key, select the Edition I want, Home , Pro etc. and bobs your uncle. I'm not sure how that works though if you have multiple installs of the same edition on one device? I don't multiboot, I only ever run one Edition on one device. I can switch editions on the fly by simply doing a change product key.