PowerPoint doesn't do shape morphing like Flash does, but Rae's idea is
excellent. You didn't say which version of PowerPoint you're using, so I'll
give instructions for 2003.
Draw an oval and choose Slide Show>Custom Animation. With the oval selected,
choose Add Effect> Exit> More Effects> Collapse.
Draw a rectangle and choose Add Effect> Entrance>More Effects>Stretch. Set
this ones Start to With Previous. Then the oval disappears as the rectangle
appears.
You could also exit the Oval with a Fade animation and entrance the
rectangle with a Fade animation.
Ellen