If you install a recent version of SIM like 7.2, 7.3 or 7.4 you have to update your agents and VCA to 7.3.x.x or higher because the agents that want to connect to the VCRM can't connect with a VCA lower that 7.3 because of security fixes made to 7.3 and above. I would recomend to update all your servers to the latest supported SPP/PSP's for the hardware you use.
That means you can't update older hardware to the latest SPP's exept for the Management software like VCA,SMH diagnostics these are hardware platform independant. SNMP Agents and WBEM agents depend on the hardware platform. Check the HP website for the latest supported SNMP & WBEM Agents supported for your hardware.