Player Modality

The switching may or not be under player control, switching could be restricted in some way. What is required is the same areas of the game world can be experienced in more than one mode, switching to another mode does not proclude switching back to the original mode, and certian interactions be exclusive to each mode. Changing the shape of the main character does not qualify by itself. The change must affect interactions so drasticly, that it seems the gameworld has changed.

Super Paper Mario is an excellent example of Player Modality. Progress in 2D mode can be blocked by objects that can simply be walked arround in 3D mode. Certian characters are 2D and cannot traverse the 3rd dimension. There are additional exclusives but just these two are enough for this game to be tagged.

Modes that might apply:
2D/3D
corporial/etherial
shrink/normal size/grow
others