Inline transform specific attributes in JSON payload containing nested arrays and objects of key/value pairs

The match operator with some recursion will be helpful here.

Try this out. I created a function, applyToValuesWhenKey, that takes in the input value as e, a function to apply to the values as fn, and a function that returns a boolean that will dictate whether or not to apply fn to the value called predicate. It uses recursion and the match operator to loop through the data, applying map, mapObject, or just returning the given value depending on whether or not the current value is an object, array, or anything else. The rest is your code:

%dw 1.0
%output application/json

%var input = {
   "key1":"value1",
   "key2":"value2",
   "arrayList1":[
      {
         "key3":"value3",
         "datefield_1":"13/01/2006",
         "datefield_2":"15/06/1980",
         "arrayList2":[
            {
               "key4":"value4",
               "datefield_3":"13/01/2000",
               "datefield_4":"15/06/2003",
               "arrayList2":[
                  {
                     "key5":"value5",
                     "datefield_5":"30/01/2000",
                     "datefield_6":"14/06/2003"
                  }
               ]
            }
         ]
      },
      {
         "key6":"value6",
         "datefield_7":"20/02/2000"
      }
   ]
}

%var keysToUpdate = [ 'datefield_1', 'datefield_2', 'datefield_3', 'datefield_4', 'datefield_5', 'datefield_6', 'datefield_7' ]

%function applyToValuesWhenKey( e, fn, predicate )
  e match {
    :array  -> $ map applyToValuesWhenKey( $, fn, predicate ),
    :object -> $ mapObject ( ( v, k ) -> { 
                             ( k ): fn( v )
                           } when predicate( k ) otherwise {
                             ( k ): applyToValuesWhenKey( v, fn, predicate )
                           }
                         ),
    default -> $
  }

%function transformDateFormat( date ) 
  date as :date { format: "dd/MM/yyyy" } as :date { format: "yyyy-MM-dd" }

%function transformDates( input )
  applyToValuesWhenKey( input, 
                        transformDateFormat,
                        ( ( key ) -> keysToUpdate contains ( key as :string ) ) )
---
transformDates( input )