What if...?

Oh, that wasn't correct.

Correct is this:

If the direction of the movement changes and if the velocity changes because of the collision is determined by the relation of the balls'. For example, one ball has thrice the mass of the other one. They both move at the same velocity. After a frontal collision we would see that heavier ball stops moving, whereas the lighter ball continues a velocity twice as high as before and that it would change its direction. The heavy ball has caused a change in momentum in the lighter ball. But it also receives momentum from the lighter ball. This means: The sum of momentum before the collision equals the sum of momentum after the collision. Altogether the momentum within the system remains the same. During an elastic collision the total amount of energy within the system must also remain the same.

Depending on how much more heavy one ball is in relation to the other, the result will vary slightly. Not always does the heavy ball stop moving but it'll always move more slowly and the lighter ball will always move faster than before.