Have you tried working the other way round?
- Draw a chevron shape: Insert > Shapes > Block Arrows > Arrow: Chevron
- Rotate the chevron so that it points down and format the shape to your liking
- Duplicate the slide
- On the first slide: click on the yellow circle of the chevron (see jpg) and pull it down as far as you can, to obtain a rectangle
- Add morph transition to the second slide
Tested this and on my screen it works out.
Have fun
