Automator :: Remove 6 digit date from filename; append current date

This should do as you requested:

#!/bin/bash

today=$(date +%y%m%d)

for f in "$@"; do

    filebasename=${f##*/}
    filename=${filebasename%.*}
    path=${f%/*}
    ext=${f##*.}

    if [[ "$filename" =~ ^.*_[0-9]{6}$ ]]; then

        filename="$(sed -E -e 's/_[0-9]{6}$//'<<<"$filename")"

        newfilename="$filename"_"$today"
        mv "$f" "$path"/"$newfilename"."$ext"   

    else    

        newfilename="$filename"_"$today"
        mv "$f" "$path"/"$newfilename"."$ext"

    fi

done

Note that when used within a Run Shell Script action in an Automator workflow, you may need to add the full path to sed in the code provided, e.g.: /usr/bin/sed