Here is how you can switch to 3.9.0 for a given virtual environement venv-name:
pip freeze > requirements-lock.txt
pyenv virtualenv-delete venv-name
pyenv install -s 3.9.0
pyenv virtualenv 3.9.0 venv-name
pyenv activate venv-name
pip install -r requirements-lock.txt
Once everything works correctly you can safely remove the temporary requirements lock file:
rm requirements-lock.txt
Note that using pip freeze > requirements.txt is usually not a good idea as this file is often used to handle your package requirements (not necessarily pip freeze output). It’s better to use a different (temporary) file just to be safe.