Animating odd shapes

Principle Version: 5.9 (5020)
macOS Version: 10.14.6 (18G103)
Sketch Version: 59.1 (86144)

Hello there,

I’m trying to animate a shape transforming into another. This worked perfectly when I drew a rectangle inside Principle and transformed it into a circle I drew in Principle (by making a rectangle with maximum radius).

This doesn’t seem to work when I upload art boards from Sketch with those same shapes. I would like to change from a rectangle to a circle to a polygon to a random shape. But since I can’t make those in Publisher and shapes from Sketch aren’t the same as Principle shapes, I don’t know how to do this.

Is this in any way possible? It would already help if at least the color transition would be recognized by Principle so it doesn’t look so ‘stiff’.

If someone knows what to do, please let me know.

Thanks in advance!