With Parallels 13, want to migrate existing MacOS 10.10.5 hard disk into a new virtual system

My wife has software that won't run if she updates from MacOS 10.10.5. Sadly, installation media for said software has disappeared.
Right now we have Parallels Desktop 13 for Mac (Home Edition) which can't be upgraded to 14 until the MacOS is upgraded.
Want to upgrade MacOS, but don't want to lose her software.

What would be great is if there was some way to:
1. create a new virtual system under Parallels 13 and migrate a perfect copy of the host system into the new virtual system
2. make sure that the new virtual system CAN run her software.
3. only then upgrade upgrade the host system to MacOS 10.13
4. upgrade Parallels 13 to Parallels 14.
5. feel safe, secure, and up-to-date

I'm having a hard time figuring out a way to do this. All the documentation I can find focuses on bringing up virtual systems running anything BUT the same OS as the host. Not surprising really, but...

Anyone have experience migrating an existing MacOS system into a virtual system?

 

No - but I did successfully install macOS 10.13 into a VM. I believe I followed this blog post: https://www.parallels.com/blogs/older-versions-mac-os-x-with-macos-sierra/

The options in this article should get you a Yosemite installer: https://macpaw.com/how-to/download-old-macos-versions