This post describes how I converted my old .sws Sage worksheets to the new .ipynb (jupyter) format.

Install the required software.
sage i beautifulsoup sage pip install git+https://github.com/vbraun/ExportSageNB.git sudo apt install python3sagenbexport sage i rst2ipynb
(I had first tried
pip install git+https://github.com/vbraun/ExportSageNB.git
in place of the second command above, but it didn't work.) 
Export the
experiments.sws
file in two steps (sws > rst > ipynb)sage sws2rst experiments.sws experiments.rst sage rst2ipynb experiments.rst experiments.ipynb
Alternatively, I think this would have worked:
sage sws2rst experiments.sws experiments.rst sagenbexport list sagenbexport ipynb=experiments.ipynb admin:0
(where
admin:0
is the unique handle to experiments.rst that showed up in the list). 
Load the experiments notebook in the new format.
Launch Sage while loading the experiments notebook as follows:
sage notebook=jupyter experiments.ipynb
Another example

Install the required software (as above).

Export the
MAADeMeoDeMo.sws
file in two steps (sws > rst > ipynb)sage sws2rst MAADeMeoDeMo.sws MAADeMeoDeMo.rst sage rst2ipynb MAADeMeoDeMo.rst MAADeMeoDeMo.ipynb

Load the MAADeMeoDeMo notebook in the new format
Launch Sage while loading the MAADeMeoDeMo notebook as follows:
sage n jupyter MAADeMeoDeMo.ipynb