What is your definition of lean? Each of the components in the MEAN stack have been tried and tested through many a applications deployed to prodcution already. Also, to answer your question- Yes, the components of the MEAN stack can be replaced.
Mongo with Couch
Express with Hapi/Koa
Angular with Backbone/Ember
Your buns are mine! But you can have this tiny ad:
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop