Billions of zombies.... that then feast on the living. This could be the worst outcome.

I think the easiest route would be to just use the built in windows disk image tool? MS have increasingly hidden it over the years but it still works. Basically you take an image of your current system, then remove the old HDD, install the new one, restore the image. It does require having enough space on a spare disk to create the image.
This guide looks to cover creating an image:
https://answers.microsoft.com/en-us/windows/forum/all/how-to-create-a-system-image-in-windows-10/84fa6683-e3ac-4e93-9139-368af9267869
and this one covers restoring it: https://answers.microsoft.com/en-us/windows/forum/all/how-to-restore-a-windows-10-system-image-to-an/e20992ca-5641-4f7c-bb09-3895d0732162

Edit: You can of course keep and reuse the old HDD. I just suggest pulling it for the initial restore if you aren't comfortable with boot settings/wiping disks etc as you might just keep booting to the old existing windows. Once the new one is setup you can then connect the old one and format it.

I'm moving to Tutanota so I have to use there client on desktop and mobile. I have Thunderbird for my Gmail and Outlook emails on Desktop and just use Gmail and Outlook apps on my phone.