Processor Upgrade for an Old Notebook?

Amphigory asks: "I have a Toshiba Satellite Pro 400CS with a Pentium 75 Mobile Processor and am trying to find out if it's possible to upgrade the processor. The problem is that I can't find any information about this processor in the obvious places. What piques my interest is that the processor reports itself as a "Pentium Mobile 75 - 200" -- does this mean I could pop a Pentium mobile 200 in the same slot and have it work? "