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In imperial Armour Apoc - Second Ed it lists the Chaos Warhound Titan as having a dirge caster.

According to these rules - the dirge caster makes all chaos units within 12" Fearless.

But the new chaos codex states a dirge caster stops units being able to Overwatch.

So would it be fair to switch the apoc rules to the current 'dex rules and say that a Titan dirge caster denies overwatch within 12"?

Nope, stick with its own version of the rules.

If it gives the rules for how the dirge caster works right there in the Titan entry, the default assumption should be to use those.