Is there a way to upgrade the version of Python used in a virtual environment (e.g. if a bugfix release comes out)?
I could pip freeze --local > requirements.txt, then remove the directory and pip install -r requirements.txt, but this requires a lot of reinstallation of large libraries, for instance, numpy, which I use a lot.
I can see this is an advantage when upgrading from, e.g., 2.6 -> 2.7, but what about 2.7.x -> 2.7.y?
Did you see this? If I haven't misunderstand that answer, you may try to create a new virtualenv on top of the old one. You just need to know which python is going to use your virtualenv (you will need to see your virtualenv version).
If your virtualenv is installed with the same python version of the old one and upgrading your virtualenv package is not an option, you may want to read this in order to install a virtualenv with the python version you want.
EDIT
I've tested this approach (the one that create a new virtualenv on top of the old one) and it worked fine for me. I think you may have some problems if you change from python 2.6 to 2.7 or 2.7 to 3.x but if you just upgrade inside the same version (staying at 2.7 as you want) you shouldn't have any problem, as all the packages are held in the same folders for both python versions (2.7.x and 2.7.y packages are inside your_env/lib/python2.7/).
If you change your virtualenv python version, you will need to install all your packages again for that version (or just link the packages you need into the new version packages folder, i.e: your_env/lib/python_newversion/site-packages)