When manufacturing, they have everything streamlined so that they use the least amount of labor/power/time to put it together.  Swapping chip, if upgrading from say Avalon 1 to Avalon 2, it might not be too bad.  If you are trying to stick chip from a different maker, then it would be massive amount of work.

It would be just like sticking a V8 in a prius as HellDriverUK said, and it might end up costing more than a Lexus in terms of work time and other costs...

Sell it and buy something new would be a better and cheaper way to go.