Increase velocity without changing trajectory

I have a rigidbody that has been launched high into the air. I've calculated the exact angle needed to hit a specific target on a distant battlefield, and it works perfectly. Now, however, I have been told to increase its velocity as it nears the ground. Is there a way to increase speed on the rigidbody without messing up the trajectory? The calculation for the trajectory is quite complex, and would be very difficult to modify directly for a sliding velocity.

Thanks! The equation I'm using is below:

 float sqrt = (v * v * v * v) - (g * (g * (x * x) + 2 * y * (v * v)));
 float theta1 = Mathf.Atan(((v * v) + Mathf.Sqrt(sqrt)) / (g * x));