Why the role-reversal was necessary.

Normally Christopher Lee plays the villains like Dracula while Peter Cushing played heroes like Van Helsing while in this film they switch sides playing the role the other normally would play.
Here's why I think it was a good idea;

In the Dracula films Van Helsing is a mortal man going up against a more powerful adversary who is faster and stronger and more vicious. So Van Helsing has to resort to cunning to out-maneuver him.
In short we have the sly, resourceful fox taking on the bigger, stronger, fiercer wolf.

But in this film the 'fox' is the villain who is trying to hide a secret and evades questions and makes up excuses.
To contrast the furtive fox the hero needed to be an aggressive, relentless 'hound', constantly sniffing his nose into others business and howling and snarling and digging until he gets results.

So this time around the Soft-spoken, sly Cushing had to play sneaky fox to Lee's barking, growling hound.