Fix bugs without breaking down existing code

[Elvis]

Say, a Standard Library function works in a way it was not supposed
to.

Developers (who use Python) handle this issue themselves.
And then, you (a python-core developer) fix the behavior of the
function.

Although you have “fixed” the bug, anyone who upgrades, will be in
trouble.
Their code may no longer work.

How do you go about this issue?
How do you fix things without breaking down existing systems?