After I launch Locust without the web UI:
$ locust -f locust_files/my_locust_file.py --no-web -c 1000 -r 100
is it possible to change the number of users or hatch rate programmatically during the execution?
1) If we want to increase the number of users during test:
Run the same test in parallel with additional number of users
2) If we want to decrease the number of users during test:
a) Run the second test with required number of users 
b) At the same time stop the first test
Both options can be automated with python or even bash scripts.
Dirty hack, but I think this will result desirable effect completely.
In Locust, can I modify the number of users and hatch rate after I start the test?