What is the code_change/3 callback used for in gen_server?

code_change(OldVsn, State, Extra) is the hook OTP calls during a release upgrade or downgrade so a gen_server can transform its existing in-memory state to match whatever shape the new code version expects, instead of losing state by simply restarting.

code_change(OldVsn, State, _Extra) when OldVsn =:= "1.0" ->
    %% old state was a plain tuple {Count}, new state is a map
    NewState = #{count => element(1, State)},
    {ok, NewState};
code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

It's only invoked as part of the relup-driven upgrade machinery, not on a normal restart, and only makes sense when a release upgrade actually changes what a process's internal state looks like — adding a new field to a record, switching representations, or renaming keys. If the state shape hasn't changed between versions, the default identity implementation (returning the state unchanged) is all you need.

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