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I'm sure this has been asked before but...

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Someone who "hacks" Tesla's like Jason Hughes (wk057 here on the forum) can probably do it using salvage motors. He did turn an original 60 into a P85. It would require changing both motors, the 60 has one large motor in the back and the 60D has two small motors. The front end would have to have some equipment added to support the front motor and extra wiring would have to be added. It would not be easy and if you paid anyone to do it, it would probably cost more than selling your current car and buying a 60D.
 
It would be about as easy and practical as upgrading a 55 inch TV to a 65.
well, there's always something easy like this ... A handy Television Screen Magnifier to double the size of your TV picture

... and ... the OP did ask if it is possible to "upgrade" (with quotation marks), so... how about : P85D 3D Car Logo Emblem Badge Sticker Chrome Letters for Tesla Model S in Tesla Model Accessories for Sale :)

but seriously, to the original question - as others have said, not practical or cost-effective, even it it were physically possible