Hi!
The change is called Denaturalization
Here's what happens: Proteins are components of egg whites. When you beat the eggs, you are adding air and the proteins expose their water-loving (hydrophilic) ends to the water and their water-hating (hydrophobic) ends to the air, creating tiny bubbles that make the cream stiff. Also, the proteins can crosslink between themselves making the foam more stable. When the structure of a protein is changed, the process is called Denaturalization.