It's not possible to modify the default configuration.

You can use the optimization.minimizer setting to instantiate your own UglifyJsPlugin, however. Using 4.0 we used this example to get source maps even when mode is set to 'production' for example (no longer necessary as of 4.1.1):

const UglifyJsPlugin = require('uglifyjs-webpack-plugin');

module.exports = {
  optimization: {
    minimizer: [
      // we specify a custom UglifyJsPlugin here to get source maps in production
      new UglifyJsPlugin({
        cache: true,
        parallel: true,
        uglifyOptions: {
          compress: false,
          ecma: 6,
          mangle: true
        },
        sourceMap: true
      })
    ]
  }
};

Without adding uglifyjs-webpack-plugin, you can just add this to the end of your webpack.prod.config.js file:

 optimization: {
   minimize: false
 }

You can try this

npm install uglifyjs-webpack-plugin --save-dev

webpack.config.js

const UglifyJsPlugin = require('uglifyjs-webpack-plugin');

module.exports = {
  optimization: {
    minimizer: [new UglifyJsPlugin()],
  },
};

webpack documentation