Dedifferentiation is a cellular process often seen in more basal life forms such as worms and amphibians.

Not necessarily, although in general a metaplastic cell is abnormal. A cell can also be metaplastic due to incomplete differentiation or due to dedifferentiation.

A tumor's grade is the term for the level of differentiation. In contrast, stage is the term for saying the extent to which the cancer has spread.

Anaplasia refers to undifferentiated cell growth in a malignant neoplasm (tumor). Lack of differentiation is considered a hallmark of malignancy. The term anaplasia literally means "to form backward." It implies dedifferentiation, or loss of structural and functional differentiation of normal cells. It is now known, however, that at least some cancers arise from stem cells in tissues; in these tumors failure of differentiation, rather than dedifferentiation of specialized cells, account for undifferentiated tumors.

To the extent that silicosis can lead to tumor production, and the formation of tumors is thought to be a stepping-stone for carcinogenesis, yes, the occurrence of silicosis can herald in cell dedifferentiation, and thus cancer.