As long as you do not need any of the new features, and the ESXi version you use is still compatible with the old hw version
there is no need to upgrade the virtual hardware.
However, I would recommend upgrading the hardware (and updating the VMware Tools which is a pre-requisite in most cases) once you have a planned downtime anyway. This way you can avoid the need for a later mass upgrade that might become necessary when a newer version of ESXi drops support for the older hw version that you still use.
- Andreas