There are several enhancements added to version 5 of VMFS. Knowing these enhancements will definitely motivate you to upgrade to this version. Nevertheless, the upgrade process is nondisruptive.
Although this is an in-place upgrade, you should take the following precautions before upgrading:
The VMware® vSphere® VMFS-5 Upgrade Considerations whitepaper explains more about VMFS-5 enhancement and the upgrade considerations. You can access the paper at http://www.vmware.com/files/pdf/techpaper/VMFS-5_Upgrade_Considerations.pdf.