If you are upgrading from a 14/2 15 amp circuit to a 12 gauge 20 amp and the current wiring is straight from the panel box to the termination point (receptacle) and does not continue on to any other loops, you luckily may be able to use the old 14/2 as a pull for the new thicker wire.
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The only problem with this is that if it is wired to code, it will be nailed at every 4 ft or so. You will have a hard time pulling that out.
But, if his outlet is the first on the home run wire, that usually is with 12ga anyhow; however, he will have to give up the other outlets but in that case, he may be in violation of code, not having them at the right places.