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The discovery, originally discussed on netkas.org forums and detailed by Chris Foresman of Ars Technica, allows users of the earlier Mac Pro models to install an EFI firmware update that essentially converts the machine into the 2010 model, albeit lacking the faster CPUs and RAM that Apple began installing with the new model.
Loading the new firmware results in Mac Pro 4,1 machines (also referred to as "Early 2009") to report they are now Mac Pro 5,1 models ("Mid 2010"). Both machines are similar in that they support Intel's new x58 chipset with QuickPath Interconnect using the Nahalem or Westmere microarchitectures rather than the Core-base Xeon CPUs of 2008 and earlier Mac Pros.
After the update is installed, it's then easy to upgrade to faster new Westmere Xeon CPUs, given that the new chips are socket compatible with the 2009 Mac Pros. Single CPU machines take standard W-series CPUs, while dual CPU models require dual-QPI enabled CPUs to function properly.
Upgrading the firmware also enables Nehalem or Westmere CPUs that can support faster 1333MHz RAM components to do so, enhancing performance above the 1066MHz limit supported by the firmware that 2009 Mac Pro models shipped with. The new firmware also enables Mini DisplayPort to deliver audio signals over HDMI.
Apple doesn't support the firmware upgrade, nor does it consider the CPU a user replaceable part; exchanging either could run the risk of invalidating AppleCare. However, upgrading the firmware is a reversible process (although some refurbished models have a specialized firmware installed that is not publicly available), and users have not yet reported any problems related to upgrading the older model's firmware.