Keras -- Transfer learning -- changing Input tensor shape
You can do this by creating a new VGG16 model instance with the new input shape new_shape and copying over all the layer weights. The code is roughly

new_model = VGG16(weights=None, input_shape=new_shape, include_top=False)
for new_layer, layer in zip(new_model.layers[1:], model.layers[1:]):
    new_layer.set_weights(layer.get_weights())