How to toggle an image beteen 3D and flat

Hello Team,

When I create a 3D image and ungroup to make a color change to a particular word or letter, the image is no longer in 3D but 'flat'

(for lack of the proper word). 

How am i able to make changes and revert back to the 3D image? or is there a better way to modify a 3D image to make color changes to a word or letter?

see screenshots illustrating the 3d & flat files.