How can I change the Python interpreter in virtual environment (Ubuntu 18.04LTS)?

As far as I can tell the venv standard library appeared in Python 3.3 and was never backported to 2.7.

venv can only create virtual environment for its own version of the interpreter and the virtual environment directory can not be moved to a different location or be renamed. Python 3.foo can not create a virtual environment for Python 3.bar. So it is best to pick the wanted interpreter right from the start.

Since, as shown by the output of whereis python, you seem to already have multiple Python interpreters already installed, you should be able to do something like the following:

$ /path/to/python3.3 -m venv /path/to/my/venvs/venv33
$ /path/to/python3.8 -m venv /path/to/my/venvs/venv38

There seems to be a way to change the Python interpreter associated with a virtual environment (I have not tested it, not sure what the limitations are):

$ /path/to/python3.8 -m venv --upgrade /path/to/my/venvs/venv33

Alternatively use virtualenv which seems to offer a bit more flexibility, but is probably less efficient (its next major release, virtualenv 20, should bring a lot of improvements though).