Here is another variation that does not used "sed -i". It uses a favourite hex editor called ired.

      test ! -h $1||exec echo $0: error: symlink
      case $# in :)
      ;;1) x=$(sed -n '$=' $1)
      test $x -gt 1||exec echo usage: $0 dst src
      od -tx1 -An|sed 's/^/w /'|ired -n $1 /dev/stdin
      ;;2) printf '0r '$2'\nwq\n'|ed -s $1
      esac