For better crew coordination in airbus SOP nothing is done quietly. Any change on the FCU is checked by PF on PFD to first confirm it is happening and called out to keep PM in the loop. If you change speed reference from managed to select or vice versa how would you keep the PM informed? You may announce speed managed or select but that does not convey you have checked it on PFD and unless you see the colour it may not have happened.