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Changing the battery should not affect a fob already paired to the car. Verify that the battery is good - new battery should be above 3.2 volts and that it is installed properly.
What post #2 said. Change the battery and it should work. Nothing else to be done.
Of course, you need to put the battery in to the FOB with the correct polarity (not upside down) and the new battery might not be all it's cracked up to be and be bad out of the wrapper.