The retroactive effect of the new provision applies to circumstances which occurred before the amendment to the notification procedure came into force, unless the tax or interest on arrears claim has already become statute-barred or has been legally established before 1 January 2011. Detailed information can be found in the FTA press release of 1 February 2017.
With regard to the repayment of interest on arrears no longer due under the new provisions, the FTA has provided for a simplified procedure in order to ensure the rapid settlement of cases and published a notice (004-DVS-2017-d - Recovery of interest on arrears in the withholding tax reporting procedure) on 1 February 2017, which determines the applicable cases and describes the procedure to be followed in order to comply with the new legal requirements.
The corresponding amendments to the law can be found here: Federal Act on Withholding Tax - Amendments of 30 September 2016.
The amendments go back to Urs Gasche's parliamentary initiative (clarification of many years of practice in the reporting procedure for withholding tax, 13,479).