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This is only a change in the interface. I don't think there is any underlying change in the notebook format. Your old notebooks are completely safe.

P.S. Juptext is a pretty cool extension https://pypi.org/project/jupytext/

nbformat is reference implementation of the notebook format: https://github.com/jupyter/nbformat No recent major releases.

jupytext is great, you can also review your notebooks as `.ipynb` files using GitNotebooks [https://gitnotebooks.com]. (I'm the solo dev)