Microsoft HyperV allows you to create 2 primary hard disk storage types, fixed or dynamic.
Dynamic size drives are allocated with the minimum space needed and space is expanded as you write additional drive to the volume. There is performance loss on disk writes that require storage to be expanded. For example if you create a dynamic volume with 10 GB and as you write data and use up the entire 10GB allocation then the volume needs to be expanded first then your data will be written to the volume. This expansion adds overhead and can slow down data writes by 25% or more.
Fixed drive sizes presets the size for your storage. Since the file is preallocated on the HyperV host there is no loss in performance due to the need for expansion of space.
The big difference between VHD and VHDX is the way this expansion is handled. In tests performed by others they have seen negligible difference in overhead with Dynamic VHDX volumes when they expand to accommodate additional data.
You can easily convert your VHD volumes to VHDX. As always, make a backup and make sure you have enough time allocated for the conversion. The larger the volume the longer the conversion will take.