53 Pa.C.S. § 8115

Except as otherwise provided in any ordinance or resolution authorizing or awarding bonds or notes or tax anticipation notes, the terms thereof and of this subpart as in effect when the bonds or notes were authorized shall constitute a contract between the local government unit and the holders from time to time of the bonds and notes subject to modification by the vote of a majority of the holders or such larger portion thereof as may be provided in the bond or note.