Have a quick look around the installation first, if it looks a bit rough and you don't want to put your name against it then walk. Always tell your customer that you will only proceed with the CU change once you have completed an EICR, if there are any faults you can then quote to repair or rewire and change the CU at the same time, or if there are no faults, you can go ahead with the swap and know there will be no issues later on. Explain this to your customer and if they don't agree to your terms then get out of there. They're either time wasters, want you to do it at foreigner prices or they've something to hide. I've had this rule since I qualified, never fails.