The current release notes, https://gojs.net/latest/changelog.html , includes the short list of incompatible changes that came with 2.0, at the end of the page.
I think almost all apps will have no difficulty in upgrading, because the incompatibilities are in infrequently used parts of the API. In general it is easy to find and replace the incompatible code, because of the nature of the changes along with the rationale for the change and how to achieve the behavior in the new API.
The most frequently noticed incompatibility is that we have taken most of the predefined Shape figures out if the
go.js library and moved them to
extensions/Figures.js. You can copy needed figures from there, or just load the whole file if you don’t mind the extra code.