As per excellent article by Derek Seaman.
Full link below :
Piece concerning VMFS upgrades below
VMFS upgrades are simple, and completely non-disruptive. You can upgrade a VMFS datastore from VMFS-3 to VMFS-5 with running VMs. However, while this may sound perfect, keep reading as the reality is more complicated. The table below shows the differences between the two filesystem versions. Now that VMFS-5 has been around for a while, I hope you don’t have too many VMFS-3 datastores around.
Ok so you are thinking, why is an upgrade not ideal? The problem is that an upgraded volume does NOT look the same under the covers from a freshly formatted VMFS-5 volume. The table below shows the differences. The most impacting can be the block size. In vSphere 4.x and earlier you had a choice of block sizes that ranged from 1MB to 8MB. If your array supports VAAI extensions the VMFS volumes must have the same block size if you are doing operations such as copying VMs. Otherwise the disk operations revert back to legacy mode and will run slower.
The VMware recommendation is to create a fresh VMFS datastore then storage vMotion your VMs into the datastore. After the datastore is evacuated re-format or decommission it. If you aren’t licensed for storage vMotion, then during your vCenter upgrade don’t input a product key. This gives you 60 days of the ‘enhanced’ license features.
VMFS will play less of a role in vSphere 6.0 and beyond with the advent of VVols. VVols does not use a filesystem, so there’s no VMFS to deal with. Once your storage array supports VVols and you migrate VMs to vVols you can forget about VMFS. I have no insider knowledge here, but I’d be surprised if VMware released any major new VMFS versions given the VVols future”