How to override character creation and make the character a cube or a part of some sort?

I'd like to make characters who spawn in my game spawn as cubes and be able to transform into triangles and spheres as well as back to a cube. I'd also like to make sure they can transform at button press.

I've had trouble finding a guide which can help me do this, anyone have any suggestions on what I should do? I'll be eternally grateful.

Shadi432 - ROBLOXian