Am not a hardware guy when it comes to Mac's. On the hardware side in the build your own world, you would not have to do a re-install (windows/linux) for anything based solely on an upgrade to the processor. In very simplistic terms, it functions as a part of the motherboard which reports to the OS what it is there, similar to RAM. Would think, hardware wise it should be the same with the Mac. Did not find any reference to having to re-install in any of the articles I found on a quick search.
However, as a hardware enthusiast, some upgrades don't really make sense. Have not done any checking, and don't know what other hardware you have in your rig or if OWC's price is about average - a $450 upgrade part to a system that old seems a bit steep.