Find all files (-type f) that have been modified more than 5 minutes ago (-mmin +5) and execute the command (exec) to replace all letters (a-zA-Z) with the number 1 (s/[a-zA-Z]/1/g) on each file ({} \;).
find . -type f -mmin +5 -exec sed -i -r 's/[a-zA-Z]/1/g' {} \;