I think it'll be all right. python3.8 is its own distinct package, which will install alongside the existing python3.6.
Debian/Ubuntu's "alternatives" system can select what /usr/bin/python3 runs. In practice, changing that away from the distribution default can break other people's code, and the win of an LTS is not spending time on that stuff.
But you can get yourself a Python 3.8, then you can build venvs, run a Jupyter kernel in it, whatever you like. If you can download or rebuild all the modules you want in a 3.8 venv, you're pretty set. And everything wanting Python 3.6 on your LTS still works.
Debian/Ubuntu's "alternatives" system can select what /usr/bin/python3 runs. In practice, changing that away from the distribution default can break other people's code, and the win of an LTS is not spending time on that stuff.
But you can get yourself a Python 3.8, then you can build venvs, run a Jupyter kernel in it, whatever you like. If you can download or rebuild all the modules you want in a 3.8 venv, you're pretty set. And everything wanting Python 3.6 on your LTS still works.