Provided likewise, and be it enacted, That where such Stipend mall, at or after the Expiration of the said Fifteen Years, be again augmented or modified by a Decree, after the passing of this Act, it shall not be again augmented or modified until the Expiration of Twenty Years from and after the Date of such Decree of Modification thereof, nor shall any such Stipend be augmented or modified, at any future Period, until the Expiration of Twenty Years from and after the Date of the last Decree of Modification thereof respectively.