Update 2017-02-14: BLT now includes a much simpler method of upgrading the VM (provided you only override VM settings in files separate from the
# Delete the entire VM and remove config. blt vm:nuke # Rebuild the VM with the latest recommended version and config. blt vm
Limiting the amount of surprises you get when developing a large-scale Drupal project is always a good thing. And to that end, Acquia's BLT (Build and Launch Tools) wisely chooses to leave Drupal VM alone when updating BLT itself. Updates to Drupal VM can and should be done independently of install profile and development and deployment tooling.