I wasn't able to create a new virtualenv on top of the old one. But there are tools in pip which make it much faster to re-install requirements into a brand new venv. Pip can build each of the items in your requirements.txt into a wheel package, and store that in a local cache. When you create a new venv and run pip install in it, pip will automatically use the prebuilt wheels if it finds them. Wheels install much faster than running setup.py for each module.
My ~/.pip/pip.conf looks like this:
[global]
download-cache = /Users/me/.pip/download-cache
find-links =
/Users/me/.pip/wheels/
[wheel]
wheel-dir = /Users/me/.pip/wheels
I install wheel (pip install wheel), then run pip wheel -r requirements.txt. This stores the built wheels in the wheel-dir in my pip.conf.
From then on, any time I pip install any of these requirements, it installs them from the wheels, which is pretty quick.