Hi Xavier,
To uninstall 2.8 DL Libraries, you can uninstall from standard windows add or remove program from the settings if you have installed from msi. If you are installing manually from conda deeplearning essential, you must have installed to your clone of arcgispro-py3 default python library, so you will just need to remove the environment and recreate a new environment once you have reinstalled 2.7.
With the recent 2.8.1, my "classify object using deep learning" issue has been resolved. It looks like 2.8.2 is about to be released, so I would stick with 2.8.1 and update the new version when it becomes available. If you need to install 2.7, you would need to uninstall Pro and reinstall 2.7 and get the installer from here.
Shingo Ikeda
Geospatial Data Scientist/Developer - Geographical Information Platform
Global Power Generation - Digital Satellite USA and Canada