One of the bigger changes that also needs to be mentioned is the introduction of Flowable in addition to Observable. In RxJava 1.0, there was only one class of Observable that combined the functionality of Flowable and Observable. It was a bit unwieldy and sometimes confusing to use and developers decided to split them apart.

Usually, it is pretty easy to migrate from an old Observable to the new one but care should be taken when the old Observable was used along with Backpressure mechanisms.

While the compilation errors about .onBackpressureBuffer() and similar methods not being found on Observable class in the new RxJava 2.0 will make it very obvious that you should use Flowable, it won't be so straightforward with the code that's written outside your application.

In such cases, there will be a need for closer investigation of whether the RxJava 1.0 Observable should be transformed into Flowable or not and will involve much more hands-on runtime testing (unless the functionality is covered by proper functional tests).