Unlike the 737, the advantages of Fly by Wire and software based design is that A 30 years old A320 fly almost exactly like a brand new A320neo rolling off the production line today. According to a Lufthansa A320 captain, there are only some minor differences between EIS1 and EIS2, some differences in backup gauges, slower respond time on the old ECAS screen. A qualified A320 pilot trained on either one can fly both with ease.

On regulatory standpoint, they share the same type rating, there isn't any requirement to transition between the a CEO and NEO beyond some computer-based type difference training on the minor in the powerplants. (Exactly what Boeing wanted to achieve with NG to MAX, and backfired)

The old A320s have been given minor software and firmware updates throughout their career, even adding features like the spoileron to some of the oldest A320s.

The upgrade is likely a way to streamline supply chains on spare parts, swapping out end of life items and bringing the whole fleet to the same standard parts number. (Can you even source a new CRT screen thesedays? It will probably cost way more than a LCD screen for the EIS2). Also quality of life improvement for the pilots, thus safety.