Can a lump be cancerous under the skin if it's been unchanged for years?

Extremely rare: By definition a malignant lesion even if slow growing has continuous mutations ongoing At one point some of the cells will produce matrix metalloproteinase which breaks down the cellular , large e-cadherins producing small e-cadherins which result in progression of cells on to the metastatic state. 2 years of dormancy with no growth would be extremely rare and bx or excision required.

Very unlikely: Show it to your physician at your next routine office visit