Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / modules / contrib / migrate_upgrade / composer.json

{
    "name": "drupal/migrate_upgrade",
    "description": "Drush support for direct upgrades from older Drupal versions.",
    "type": "drupal-module",
    "require-dev": {
        "drupal/coder": "^8"
    },
    "license": "GPL-2.0+",
    "require": {},
    "minimum-stability": "dev"
}