Hi @Pavel_Kozlov and welcome to the forum!
You’re correct that
stories do contain information about changes to custom fields.
However, rather than trying to use the stories API endpoints in themselves to achieve your goal, you’re probably better off using Events. A change event on a task will tell you the old and new values of the custom field that changed, so that should give you the exact information you need.
Take a look at the API docs here:
You’ll probably have follow-up questions; feel free to ask them!