That’s great! Thanks for testing!
No easy way, though JupyterLab itself should refuse to install non-compatible extensions and will try to find compatible extension versions. We’ve been encouraging people to port extensions to JLab 2.0 rc to check the upgrade process. I imagine we’ll get a lot of extension releases next week after 2.0 is released, since most extensions that are being updated won’t ship a final version until jlab 2.0 is released. ipywidgets will release a compatible version next week, for example, but for now, you’d have to install the ipywidgets jlab extension prerelease:
jupyter labextension install @jupyter-widgets/jupyterlab-manager@next