vhdx to vhd conversion is certainly possible and you can do this disk image conversion right in Windows. The fastest way how to convert vhdx to vhd would be to use PowerShell and execute the following command:
PS C:\> Convert-VHD –Path c:\test\MY-VM.vhdx –DestinationPath c:\test\MY-NEW-VM.vhd
But you can also find dedicated utilities designed exactly for the purpose of exporting vhdx files to vhd or vice versa.
Open VHDX file Open VHD fileA virtual hard drive disk converter