Yes, W-10 sets Windows Mail to the level it was when it came out of MS.
I did all the conversion (tweaks) to make it work in Windows 7, 8 and 8.1, then copied all the files in the Windows Mail program folder, in 8.1, to a folder on my Utilities disk (& Flash Drive).
After W-10 has done its worse, I go in and with "Grant Admin Full control" running, I take ownership of all the files in the Windows Mail folder and then delete them (all).
Then I copy all the Windows Mail files that I have stored on my Util's disk and paste them into the Windows Mail folder on the hard drive. Then I right-click on WinMail.exe and 'Send to Desktop, & create a shortcut". Job done.
I can then import my saved address book and saved email folders. I've gotten the process down to less than five minutes.
* It's important to always work with the files in the "Program Files" folder, regardless of whether you're running the 32 bit or 64 bit version of the OS.
I can't pin down the exact file, but there is a difference between the files in the 32 bit version of Windows and the files used in the 64 bit version. I have them both, in two different folders, appropriately named.
What works for Windows 8.1 also works for Windows 10. I've done it many times so far.
good luck
TechnoMage