Q Does Apple ever offer processor or motherboard upgrades for desktop Macs? If not, why not?
A I cannot recollect any Mac having an official processor or motherboard upgrade available, although there have been several third-party offerings that are discussed in detail at xlr8yourMac, which also discusses unsupported Intel processor swaps that have been tried experimentally.
Although widely thought of as a software company, Apple has always taken hardware engineering very seriously. Each Mac is designed to match its hardware capability, so that if you tried swapping the processor in an old Mac Pro for a modern faster one, then its system bus, hard disk interface, and other components would need upgrading too, as they would then limit its performance.
In addition there are several custom chips that would need upgrading, to ensure that the end product worked properly. Apple has also long used surface-mount rather than socketed components to reduce production costs and improve reliability, making simple chip swaps practically impossible in most models. So the situation is not as simple as with commodity PC components, and upgrades would save little cost, and compromise the Mac’s ‘plug and play’ philosophy.
Updated from the original, which was first published in MacUser volume 26 issue 25, 2010.