No, the plan is bound to whatever machine is listed on the AppleCare confirmation (Which you should have received when you either bought the machine, or when you registered it with AppleCare.)
If you sold the 12" that had AppleCare, you can transfer coverage to the new owner by informing Apple (http://www.apple.com/legal/applecare/app.html, section 9) but you can't transfer it to a different system.
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