Apple - How to replace a folder that's name is a date i.e. YYYYMMDD with folder hierarchy of year, month, date?

You can use the following in Terminal. cd to the containing folder, then run the following:

find . -type f -exec bash -c \
  'F=$(sed -E "s#^\./([0-9]{4})([0-9]{2})([0-9]{2})#\1/\2/\3#" <<< $1);\
  mkdir -p -- $(dirname "$F");\
  mv -- "$1" "$F"' - {} \;

find . -type f obtains every file in the current directory recursively.
-exec bash -c opens a shell to run the following commands.
F=$(…) opens a subshell and uses sed on the file path to manipulate the path into the folders.
^\./([0-9]{4})([0-9]{2})([0-9]{2}) is a regex with three capture groups, as follows:
\1/\2/\3 is replacement, where each capture group (\1, etc) are separated by /.
mkdir -p -- $(dirname "$F") creates the directories to move the files into.
mv -- "$1" "$F" moves each file into its corresponding folder.

This takes the hierarchy on the left and converts it to the hierarchy on the right:

├── 20170201               └── 2017
│   └── abcdefghij             ├── 02
└── 20170302                   │   └── 01
    └── abcdefghij 2           │       └── abcdefghij
                               └── 03
                                   └── 02
                                       └── abcdefghij 2

If there are other files in the containing folder with a date as a name, they will be moved as if they are a folder. To prevent this, replace the second line with:

  'F=$(sed -E "s#^\./([0-9]{4})([0-9]{2})([0-9]{2})(?:/.+)#\1/\2/\3#" <<< $1);\

The (?:/.+) ensures that the path has a subsequent component, therefore ignoring anything without a child in the parent directory which are files.

Assuming all these YYYYMMDD folders are part of the same parent directory you could run

cd PARENT_DIRECTORY
for d in */; do
    [[ $d =~ [0-9]{8}/ ]] || continue
    mkdir -p -- "${d:0:4}/${d:4:2}"
    mv -- "$d" "${d:0:4}/${d:4:2}/${d:6:2}"
done

• The for d in */; do loop reads all directory entries, the trailing / ensures that only directory names actually match
• [[ $d =~ [0-9]{8}/ ]] tests whether the current entry consists of 8 digits, and continues with the next entry if not
• ${d:0:4}/${d:4:2}/${d:6:2} uses parameter expansion within bash to create a string containing the new path
• The -- in both mkdir and mv prevents problem in case the directory or file name starts with a -. This can't happen here but it's probably good practice anyway.

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Thanks to @terdon and @user3439894 for ideas on how to improve the original script.