There is no backward compatibility

To answer this heading, no, we don't have backward compatibility. If you have some Angular projects implemented with the previous version, depending on the complexity of the project, I wouldn't recommend migrating to the new version. Most likely, you will end up hammering a lot of changes into your migrated Angular project and in the end, you will realize it was more cost effective to just create a new project based on Angular from scratch.

Please keep in mind that the previous versions of Angular and Angular share a name, but they have huge differences in their nature and that is the price we pay for a better performance.