Ember CLI and Ember.js have some build time issues in older versions. One often mentioned solution is to upgrade stack to new versions. It means updating whole stack.
What is the best strategy to upgrade it to not destroy the whole project?
Use ember-cli-update. The README includes instructions on how to use both the global npm package version as well as the addon version. It also has an option to run codemods, to easy upgrading.
I recommend moving from LTS to LTS at a maximum, because you are likely to surface deprecations in your project's dependencies.