The rent may be altered by agreement between the landlord and the crofter to such amount and for such period as may be agreed on; and the rent so agreed on shall be the rent payable by the crofter so long as such agreement subsists, and after the expiry thereof so long as no different rent shall have been fixed by the Crofters Commission upon the application of the landlord or the crofter, and so long as no new agreement between the landlord and the crofter shall have been made.