Godot currently has a bug that prevents altering dictionaries made via json_parse; this is one user's workaround. Might come in handy considering I'm using json as my godot project's data format.

copy_dictionary_recursively.gd

# Workaround proposed by Hammer Bro again :

const DICTIONARY_TYPE = typeof({})

# Recreate the structure of a dictionary, maintaining references to all leaf-level values.
func copy_dictionary(dictionary):
   var output = {}
   for key in dictionary.keys():
      var value = dictionary[key]
      if typeof(value) == DICTIONARY_TYPE:
         output[key] = copy_dictionary(value)
      else:
         output[key] = value
   return output
   
# So that in previous code, one need to call :

    json_dictionary.parse_json('{"outer_key" : {"inner_key" : "value"}}')
    json_dictionary = copy_dictionary(json_dictionary)
    
# Basically, this workaround copies the content of the dictionary obtained by parse_json() into a new dictionary structure.