I wouldn't call Ariane 5 an evolution of Ariane IV.

Code and digital system re-use in aerospace systems is not uncommon. After all, the fly-by-wire computer system on board the Space Shuttle was derived from the original Apollo flight computer, and they are two very different space vehicles..

Right but the point is it doesn't let you make assumptions that tests aren't needed, just that you expect them to be likely to pass. The design still has to be tested as though its a new system, it's just the re-use hopefully saved some development time and the testing hopefully finds fewer issues.