Three years after the second season of Batman: The Animated Series ended production, the show was moved from Fox to The WB network, which was airing and producing Superman: The Animated Series. These shows were merged as part of an hour-long segment called The New Batman/Superman Adventures. The WB wanted more episodes of Batman, so 25 new episodes were produced, which featured a different format and more focus on Batman's supporting cast.
In addition to the network's demands, the producers decided to make the show match the graphic style of Superman, so all the characters and objects were redesigned as more "animation friendly" with fewer lines, usually referred to by the fans and creative staff as the "revamp" (or alternately, the "new look"). A similar graphic style was used in the rest of the DCAU later on.
The DVD box set of the series is labeled Batman: The Animated Series – Volume Four (from The New Batman Adventures), most likely to establish the connection with the original series.