rigidbody.velocity is a vector - its direction defines direction of movement and its length defines speed of movement. Using these properties, you can:

1. Save up the velocity length (float cachedLength = rigidbody.velocity.magnitude;).
2. Apply the force you want to modify the direction
3. Change velocity length to maintain the speed: rigidbody.velocity = rigidbody.velocity.normalized * cachedLength;

This way, you accept change in direction, but not in legth of the vector.