- draw a circle with 8 vertex instead of 4 (use the spline points settings of the circle tool parameters panel)
- draw two horizontal guidelines that represent the top and bottom of the second shape
- split the tangents of the (4 extra) spline points that are crossed by the guidelines
- got to animation mode
- go to frame 24 and change the shape of the circle using the guidelines
- press play…
Not complicated at not more work at all. I think it takes longer to start up Inkscape and draw a circle there and export it to Synfig. 
circle.sifz (1.08 KB)