Now that I have a complete update to the source code version
(I decided to throw out the one I was working on and start over completely from scratch),
I am trying to put the script in.
I would like to experiment with the
"after the projectile is fired, the keystrokes change the trajectory of the projectile,"
but is something like the following code sufficient?↓

// ...Existing variable definitions...

// New variables for trajectory change control
private bool hasChangedDirection = false; // Indicates whether a trajectory change has already been made
public FPVector changeDirectionVector; // Direction for the trajectory change

// ...Existing method definitions...

void Awake() {
    // Set the direction for trajectory change (e.g., 45 degrees upwards to the right)
    changeDirectionVector = new FPVector(1, 1, 0).normalized;
}

public override void UFEFixedUpdate()
{
    if (!this.isActiveAndEnabled || destroyMe)
    {
        return;
    }

    // Detects key input for changing trajectory (e.g., Space key)
    if (!hasChangedDirection && Input.GetKeyDown(KeyCode.Space)) {
        hasChangedDirection = true;
        ChangeProjectileDirection();
    }

    // ...Contents of the existing UFEFixedUpdate method...

}

// Method to change the trajectory
private void ChangeProjectileDirection() {
    movement = changeDirectionVector * data.speed; // Update the trajectory with the new direction and speed
}

// ...Remaining method definitions...