I know biopsy is warranted but does increased growth & slight colour change to dysplastic nevi always mean cancer?

Not always: It must have a biopsy to confirm but sometime the blood supply to the area can cause color changes. Would rec a deep biopsy to ensure specimen is good to eval and if positive would rec a surgeon (not derm ) to remove with lymph node sampling.

Need biopsy: I think you answered your own question. Changes require further evaluation. A diagnosis can only be made through biopsy.

No: However, you are correct in that a biopsy or removal may be warranted.