Could you let me know how to check how many users converted from one role to another for a specific period of time (for example how many changed roles from user to superuser from 01/09/2017 to 01/02/2018). How to track user role field change?
Drupal doesn't record when a role was changed for a user, at least not out-of-the-box.
A module that can help is the Role Watchdog.
Or if you have a store like Ubercart where you sell access, or have expiry dates for roles you can track role changes through available tools there. But it would be overkill to use a store just to track role changes, unless you already have a store installed.