Changing grunt uglify config options on watch - gruntjs

Question

I'm trying to alter my default uglify dev options on 'watch', however they're not getting set. I'm planning on adding more subtasks to the uglify task for 'production' as an example, which will use the default options, however for my 'dev' subtask, i'd like to pass new options with the watch task.
module.exports = function(grunt) {
// Project configuration.
grunt.initConfig({
// ...
uglify: {
// default options
options: {
banner: '<%= banner %>/* Built <%= grunt.template.today("yyyy/mm/dd") %> for <%= powerful.name %> */\n',
preserveComments: 'some',
report: 'min',
mangle: {
except: ['jQuery', 'Backbone']
}
},
dev: {
// options were here...
files: {
// Powerful Theme JS
'<%= powerful.jspath %>touch.min.js':
[
'<%= powerful.jspath %>libs/jquery.hammer.js',
'<%= powerful.jspath %>dev/touch.js'
],
'<%= powerful.jspath %>768down.min.js':
[
'<%= powerful.jspath %>dev/768down.js'
]
}
},
production: {
// production files here, use default options....
}
},
watch: {
js: {
files: ['<%= themesPath %>/**/js/{dev,libs,bootstrap}/*.js'],
tasks: ['uglify:dev'],
options: {
nospawn: true
}
}
}
});
// ...
grunt.event.on('watch', function(action, filepath) {
grunt.config.set('uglify.dev.options', {
banner: '/*! TESTING 123 */',
preserveComments: 'all',
report: false,
compress: false,
beautify: true
});
});
grunt.registerTask('default', ['watch']);
};