How is your vmware host set up? Is it in a cluster with the VMs stored on the SAN? Where's the ESXi local disk?
If you're in a cluster this can only be done non disruptively if your VMs haven't had their hardware version upgraded. If your hardware version for the vms is 8 or higher you will not be able to non disruptively back your vm down, and I wouldn't recommended trying to manually image disks. It's far too risky, especially with instructions from me in a forum post. ;)
http://kb.vmware.com/kb/1003746
If the hardware version isn't 8 or later, you can put the host into maintenance mode, grab a cisco edition ESXi disk from VMware for 5.0, and reinstall ontop of the existing setup. Make sure to select the ESXi boot disk and not your VMFS san luns!!!
Once esxi is reinstalled, reconnect it in vcenter, reconfigure as necessary, move VMs back.
I would personally not bother. If the fans aren't causing an issue besides for noise just hang tight and do a firmware upgrade when possible. This is far more risky than getting the right firmware.