The obsolescence problem (component life of 7 years, compared to aircraft life of 30+ years) whilst maintaining high-capability and low–cost upgrades is the holy grail for avionics systems integrators. The use of COTS and getting maximum reuse of existing functionality is key to solving this problem. However, such an approach can compromise the certification basis of the aircraft. The originally generated and gathered evidence alongside the reasoning justifying the (safety) reliance on the system, are invalidated by the change introduced to the system. This paper, based on an industrial case-study, describes a number of steps necessary to establish the assurance of safe system operation that will be considered acceptable to all involved stakeholders, such as the developers, the customer and the relevant regulatory authorities. Finally, the paper demonstrates how the reasoning behind the safety acceptance of the system is presented using argumentation; a very popular approach to represent safety cases, which document and communicate the safety reasoning of a system, and constitute a legal requirement in the UK for all safety implicated systems.