Section 7. If a change in use of land actively devoted to agricultural, horticultural or agricultural and horticultural use occurs between October first and June thirtieth of the year preceding the tax year, the board of assessors shall disallow or nullify the application filed under authority of section six, and, after examination and inquiry, shall determine the full and fair value of said land under the valuation standard applicable to other land and shall assess the same according to such value. If, notwithstanding such change of use, the land is valued, assessed and taxed under the provisions of this chapter in the ensuing year, upon notice thereof said board shall enter an assessment and the amount of the increased tax resulting from such assessment, as an added assessment and tax against such land, in the ''Omitted list'' for the particular year involved in the manner prescribed in section seventy-five of chapter fifty-nine. The amount of the added assessment shall be equal to the difference, if any, between the assessment imposed under this chapter and the assessment which would have been imposed had the land been valued and assessed as other land. The enforcement and collection of additional taxes resulting from any additional assessment so imposed shall be as provided by said chapter fifty-nine. The additional assessment imposed under this section shall not affect the conveyance or roll-back taxes, if any, applicable under sections twelve and thirteen.