HMRC has amended its guidance page, https://www.gov.uk/guidance/gift-aid-declarations-claiming-tax-back-on-donations, on gov.uk to make clear that they accept that charities and CASCs can use up old stocks of gift aid declaration forms after April 2016, if they were ordered and printed before HMRC announced its new declarations in October 2015. 

The amended text from HMRC's webpage is set out below. Charities which choose to use up old stocks after 5 April 2016 should be sure that they can demonstrate, if challenged by HMRC, that the stocks were ordered and printed before 21 October 2015. Charities are not obliged to use HMRC's model declarations, but, as HMRC's webpage states, "if you do, you can be sure that declarations will meet HMRC’s requirements."