It won't happen all at once. Linux already coexists with Windows through WSL and virtualization. The real work is in porting the APIs that the core MS products make heavy use of (DirectX, .NET, GDI, etc.).
The old Windows kernel will run in a hypervisor as needed. Windows already takes advantage of virtualization for everyday programs for security purposes. It wouldn't be that much of a shift to virtualize the kernel too.
More and more of the MS stack is being ported to Linux, seemingly in preparation for this. As of recently you can run PowerShell on Linux and run .NET "Core" on Linux. Since Visual Studio and Office are all .NET, if the Windows graphics/GDI layer is also ported to Linux, then it should be possible soon to build Microsoft Word for Linux.
