Mont. Code Ann. § 77-3-437

The owner or owners of any existing state oil and gas lease shall have the right to exchange it for an amended lease containing the same provisions as to the term of said lease as are then provided by law for state oil and gas leases upon filing a written application therefor and upon payment of a sum which is the full market value of the exchange, as determined by the board. The amended lease shall be subject to all other provisions, including rents and royalties, contained in the original lease and shall bear the same commencement date as the original lease.