Example to convert from a dynamic to a fixed VHDx file:
Convert-VHD -path "C:\Hyper-V\server-1.VHD" -destinationpath "C:\Hyper-V\server-2.VHD" -vhdtype fixed
Once the VHDx has been altered the new file will not contain the correct permissions. If you rename the new VHDx to the same as the old file (after renaming the old one) then the Virtual Machine will not boot. If you examin the permissions of the two files you will see the differences. The simplest way to correct the permissions is to add the new VHDx through Hyper-V manager (or if you have renamed the VHDxs simply remove and re-add the Hard Drive in Hyper-V manager).