The update from AngularJS to Angular 2 was a huge step forward. Still, it was also infamous for not being backward compatible. In contrast, Angular 4 brought us new features while being backward compatible. And now, six months later, the latest major release of Angular 5 is available.
The upgrade to Angular 5 is an incremental one. Because many of the changes re done invisibly.
However, some breaking changes included, so some developers will still have a lot of work to do. The changes will be easy to handle for all the developers who kept their codebases up-to-date.
The focus of Angular 5 was to make Angular faster again. Smaller bundles have improved the loading time. The execution has been developed as well by optimizations at the compiler. For some projects, it is also important to note that the ng-upgrade is still in progress.
AngularJS applications allow being migrated smoothly to Angular and vice versa as Angular components embedded as downgrades in AngularJS applications.
Additionally, experimental technologies and features developed in a unique lab space outside the Angular Framework. They can be evaluated and tested by developers at an early stage without negatively affecting the release planning of the framework itself.