If I update for example G-???? easyjet to become G-???? Monarch all I do is change the Reg and Airline in database explorer, I assume the Mode S code does not change.
However if I change G-???? BMI to SE- City Airlines, does the Mode S code change? as it is now registered in a new country.
Therefore if I dont have the new Mode S code , am I wasting my time changing Reg and Airline and would I be best deleting the old record and wait for it to update next time it shows up on RB.
As you have hinted at, the answer to your question varies from country to country.
The ICAO rules say that as long as an aircraft stays on a particular country's register (G-, N, etc) it should keep the same 24-bit ICAO address (aka hex code).
Few countries actually operate that way (although the UK does, in fact).
Many other countries (USA, Canada, Germany, etc) have predefined tie-ups between all allocatable registration marks (whether in use or not) and a corresponding address. This means that if an aircraft moves to a different registration within a country's registry (e.g. from one N number to another N number) the hex code will change.
In either case, you can't create or change a record in your NavData database unless you know the new hex code. And if, over time, you pick up more than one aircraft with the same registration/hex code (e.g. re-use of an N number, which is fairly common) you will have to choose which details to store, since none of the databases that come with enthusiast Mode S receivers can handle multiple uses of a hex code.
HTH
Dave