That's why the fact that these subs are getting an overhaul should be worrying.Most folks are just focussing on the facts that new missiles will be fitted,which is serious enough.But whenever subs have to be drydocked,there is always the chance of upgrading it's sonars,EW & countermeasures systems & also the possibility of installing new batteries.Iran reportedly had problems with the Kilo's batteries when they entered service.Moreover,a new coating of anechoic tiles may be added for absorbing sonar pings.