Parent beat me to the one line solution, but perhaps a minor mod may be needed; I suspect that the OP may have numbers larger than just 1-6, so perhaps: 
perl -pi -e 's/^(\d+)/chr($1+96)/e' myfile

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In reply to Re^3: Efficient way to replace a set of values with another set of values by space_monk
in thread Efficient way to replace a set of values with another set of values by darklord_999

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