Give an example of changes that occur during differentiation in a multicellular organism

I think you're looking for Cellular differentiation

Cell differentiation or cellular differentiation refers to the procedure via which a cell goes through the modifications in gene expression to turn into a more unique kind of cell. It is the procedure where the cell modifies from one cell type to another, generally, the cell changes into a more unique kind.

The procedure of cell differentiation permits the multicellular species to develop specifically functional kinds of cells and body plans. The process of cellular differentiation allows the formation of tissues as distinct as muscle and brain tissue from a single cell.