Do you know if it's easy to start using pyenv with existing projects, or should I wait until my next de novo project?

It’s pretty easy if you have a set of requirements handy and have `pyenv-virtualenv`:

    $ pyenv virtualenv 3.6 some-name
    $ echo some-name > /path/to/project/.python-version
    $ cd /path/to/project
    $ pip install -r requirements.txt

That’s it.

Curious here... why would you use it over just venv (assuming python3)?

Pyenv is a different thing than venv, it manages versions of Python itself rather than isolating dependencies for projects. You're probably confusing it with pyvenv, which is Python 3.4+'s command-line interface for managing venv-compatible environments.