The DoA can be used to pursue other forms of misselling in the future even if the IVA is closed. You would not need to be involved so no reason not to sign it. You would need to ask DFD why there are changes to the terms and conditions and how they would impact on you.
I'm just puzzled on why they would change the terms and conditions 6 months before completing the iva when I spoke with them they said I needed to sign both but if this is the case why 2 separate forms