How To Convert VHD Virtual Disk To VHDX Format

The simple way is to use powershell cmdlet: Convert-VHD

The Convert-VHD cmdlet converts a virtual hard disk file by copying the data from a source virtual hard disk file to a new virtual hard disk file of a specified format and version type. The format is determined by the file name extension of the specified files, either .vhdx or .vhd. Conversion is an offline operation; the virtual hard disk must not be attached when the operation is started.

EXAMPLES

Example 1

PS C:\> Convert-VHD -Path c:\test\testvhd.vhd -DestinationPath c:\test\testvhdx.vhdx

This example converts a source VHD to a destination VHDX. Because the format is determined by the file name extension and the default type is determined by the source virtual hard disk when no type is specified, the destination virtual hard disk will be a VHDX-format disk of the same type as the source virtual hard disk.

Example 2

PS C:\> Convert-VHD -Path c:\test\child1vhdx.vhdx -DestinationPath c:\test\child1vhd.vhd -VHDType Differencing -ParentPath c:\test\parentvhd.vhd

From technet https://technet.microsoft.com/en-us/itpro/powershell/windows/hyper-v/convert-vhd