1. Since I am building it from scratch would it be wise to stick to RS 485 and only to change connector type, let say to terminal block for RS485, and to use separate cable for power.What would be your choice for addressing, 8 pin DIP switch or to write address directly in memory?
2. In future time they will change software and it will be made using protocol I will suggest, so for now I can only think of modbus, only because it has CRC and display (slave) can send feedback so master can check weather data displayed is correct. I don't see much of modbus examples, so I am bit worried because I am unfamiliar with it. 3. What speed could be recommended as "safe"?
4. And finally, for power, they used 7805 as power regulator, I would prefer a switching one and that is not my field at all so, if you have some ideas, I would like to hear them.
...or MAX7219 or similar.One chip, and a decoupling cap for each 8 digits... away you go.
david_2018 said why wait software change, well simply, tight budget. A large portion of software will need to be replaced and that costs, so for now it is delayed for next year or two.
... wehere "arduino" does not necessarily mean "Uno". You could use ESP8266 etc. if wireless is needed