Section 20. This chapter shall not, except as herein expressly provided, affect the right to keep up, maintain and use any lawfully existing water mill and mill dam; but if the owner or occupant thereof makes any material change by raising the dam or by altering the machinery or the manner of using the water, so as to cause additional damage to the land of another, it shall be considered a new mill or dam, in respect to such additional damage, and the remedy for compensation therefor shall be substantially the same as in the case of a new dam.