The actual physical swapping out is usually pretty easy, unscrew the terminals, unclip from the rail and remove then do the opposite to fit the new breaker.
The circuit should be checked to see if it still meets earth fault loop impedance requirements (it almost certainly will but it should still be verified) with the C6 breaker..
Also afaict changing the protective device charateristics on a circuit is notifable work.
Or more accurately, tests are needed to show that the circuit would be OK with the different breaker. There's no point not doing these before changing the breaker since the result may be that you can't change it.
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