Hyper-V manager has a quick tool to convert Hyper-V’s vhdx format to vhd format. It is available in the actions pane called “edit disk”. Among the various options there is one which will let you convert the vhdx to vhd format.
There is also a PowerShell cmdlet which is reported to do the same thing.
By the way, I am trying this conversion to see if I will be able to import a VM from Hyper-V to VMware. Result: Nope, it does not work At the time of importing, VMware client throws up an error.
So I am wondering why would one want to do the vhdx to vhd conversion anyway? Maybe if you want to send a Hyper-V 2012 VM to someone who has Hyper-V 2008.