can we replace an existing node with a better hardware,

@pjoy 

• Nodes that use different CPU generations cannot be mixed in the same block. So a -G4 block must contain only -G4 nodes, a -G5 block must contain only -G5 nodes, and so on.
• Nodes that use different CPU generations can be mixed in the same cluster. No additional configuration is required in AOS or AHV, but 3rd party hypervisors require the configuration of CPU masking in order to support live migrations of VMs across nodes of different generations.
• Mixing nodes within the same cluster from multiple hardware vendors is not supported. However, you can manage separate clusters using the Prism/Prism Central web console regardless of the hardware/vendor type.

More details > 

https://sizing.readthedocs.io/en/latest/discovery/ntnx/ntnx.html#mixing-nodes

HTH

Vignesh