code: @Override
      public void preprocess(ServiceModel serviceModel) {
  
          if (renameShapes == null || renameShapes.isEmpty()) return;
          // sanity check
          for (Entry<String, String> entry : renameShapes.entrySet()) {
  
              String originalName = entry.getKey();
              String newName = entry.getValue();
              Shape originalShape = serviceModel.getShapes().get(originalName);
  
              if (originalShape == null) {
                  throw new IllegalStateException(
                          String.format("Cannot find shape [%s] in the model when processing "
                                  + "customization config renameShapes.%s", originalName, originalName));
              }
              if (serviceModel.getShapes().containsKey(newName)) {
                  throw new IllegalStateException(
                          String.format("The shape [%s] for the new name is already in the model when processing "
                                  + "customization config renameShapes.%s", newName, originalName));
              }
          }
  
          for (Entry<String, Shape> entry : serviceModel.getShapes().entrySet()) {
              String shapeName = entry.getKey();
              Shape shape = entry.getValue();
  
              preprocess_RenameMemberShapes(shapeName, shape);
          }
          for (Operation operation : serviceModel.getOperations().values()) {
  
              if (operation.getInput() != null) {
                  preprocess_RenameInputShape(operation.getInput());
              }
              if (operation.getOutput() != null) {
                  preprocess_RenameOutputShape(operation.getOutput());
              }
              if (operation.getErrors() != null) {
                  for (ErrorMap error : operation.getErrors()) {
                      preprocess_RenameErrorShape(error);
                  }
              }
          }
          for (Entry<String, String> entry : renameShapes.entrySet()) {
              String originalName = entry.getKey();
              String newName = entry.getValue();
  
              Shape shape = serviceModel.getShapes().remove(originalName);
              serviceModel.getShapes().put(newName, shape);
          }
      }
docs: Rename shapes for Member, Input, Output, ErrorMap, as well as the key for two maps:
  serviceModel.shape() and shapeModifiers