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$ sed -e 's/\(.\{16\15\}\)\([0-9]\{2\}\)\([0-9]\{2\}\)\([0-9]\{4\}\)/\1\4\3\2/g' /tmp/test.txt
seems to do what you are expecting.
$ sed -e 's/\(.\{16\}\)\([0-9]\{2\}\)\([0-9]\{2\}\)\([0-9]\{4\}\)/\1\4\3\2/g' /tmp/test.txt
$ sed -e 's/\(.\{15\}\)\([0-9]\{2\}\)\([0-9]\{2\}\)\([0-9]\{4\}\)/\1\4\3\2/g' /tmp/test.txt
