AngularJS falls out of support at the end of this year. If you still have a sufficiently large AngularJS codebase then a full migration is not an option. There are third party companies that sell support for when the official support ends, if being supported is important.
The only viable option is to move to a Hybrid application mixing AngularJS with Angular. This involves the use of ng-downgrade and ng-upgrade to bridge the boundary between the two.
The first stage of this process is to get the Angular JS app upgraded to the latest version. This can be non-trivial if you have an old codebase.
Early versions of AngularJS used an older proposal for Promises that predates (and is incompatible with) the now standard Promises. This may require the team to learn the differences between old and new Promises.
Modern Angular libraries now use Observables. This is another technique to understand before migrating. Observables allow the consumption of data before it has been fully read. This typically allows pages of data to be consumed. However with an older codebase for the backend then this may not be very useful.
The new libraries assume that the data being supplied is JSON, rather than the XML that older applications used. Early AngularJS projects will probably have a Java backend based upon the technology recommendations of the time.