Hi NJMorf, welcome to the Ten Forums.
First the bad news, then an explanation: There's absolutely nothing
¹ you can do to save your Insider activation for that deleted vm. Whatever build you will install on a new vm, you will need a valid product key to activate it. In other words you need to buy a Windows 10 license for the vm.
All Windows computers, be it a real physical PC or a virtual machine have a
unique ID and signature. When you activate Windows 10 on any machine (again: a real one or a vm), the ID and signature of that machine will be stored on Microsoft activation servers. You can reinstall Windows 10 on that same machine without entering a product key because when online, Windows contacts those servers and checks that the machine has a so called
digital entitlement to be activated.
Two screenshots to explain the above. First the latest
Build 14251 on a Hyper-V virtual machine, clean installed. I had previously activated
Build 11102 on this vm, let Windows Update download the build upgrade, created a Build 14251 ISO image and clean installed on top of the previous build without entering the product key. Because the vm ID and signature matched those stored by MS, the fresh installation was automatically activated:
Using the same ISO image, clean installing on a new vm, the machine gets totally new ID and signature. According to MS activation servers this machine has never been activated, therefore it will not be activated before I enter a valid product key:
Deleting the vm when you upgraded your machine is in effect the same as if you had completely dismantled a PC, poured acid over all components and brought them to trash. The machine, its ID and signature, its digital entitlement are all gone to Cyber Heaven and there's no way to restore them.
To avoid this in the future export your virtual machines before wiping the disk and reinstalling Windows. You can also use normal system imaging tools to create a system image of your vm, reinstall Windows and your virtualization software, create a new vm making it boot from the boot disk of your chosen imaging software and restore the image.
Kari
¹ = For those wanting to post something clever, I am talking about legal, valid activation methods according to the Windows 10 EULA.