Existing cells don't actually dedifferentiate, but new, neoplastic cells are less differentiated than normal cells. In case of malignancies, degree of anaplasia determines degree of malignancy, i.e. less differentiated cells are typically indicative of a more malignant neoplasm.
Change from one cell type or tissue type to another as a result of physical or chemical stress to a tissue.
Simple epithelia can become stratified in response to physical stress
Chronic gastric reflux into lower esophagus causes the stratified squamous epithelium to become a gastric-like epithelium that secretes mucus (Barrett's epithelium). The mucus protects against acid and pepsin.
Classification of Epithelial Tumors
Benign 1. Papilloma
Malignant 1. Carcinoma - many types (i.e. adenocarcinoma)