What are the means by which benign tumors become malignant?

They usually don't: Most benign tumors do not become malignant unless there has been some change in their environment. Tumors can only be diagnosed as benign with a biopsy. If benign, following size and change in shape or consistency may be used to raise concern. However, most benign lesions stay that way. If changes occur, see your doctor again --- and a biopsy is the only way to make the right diagnosis.