Well,
If you can do anything to an obtuse triangle, you could add a side and make all the angles right angles.
If all you can do is alter the angles, you will not be able to do that because you will still have a triangle with three sides.
If all you can do is alter the number of sides, you will still not be able to do that because you will have some type of quadrilateral, not a rectangle.
In essence, you must be able to both change the angles and the number of sides in order to make any triangle a rectangle.