No doubt BA would be quite happy with that in theory.
Unfortunately for them, the pilot's Scope agreement (which is now legally binding!*) means that if the RJs are replaced with aircraft designed to have more than 100 seats, whether they are fitted or not (the A318 is designed to have a minimum of 108), then all the flying must revert to BA mainline pilots from Cityflyer pilots. What that would likely mean in practice is that the current Cityflyer pilots would transfer to BA mainline with all that entails (the option to bid onto longhaul fleets for example). However, as the mainline pilot cost-base is likely to be higher, along with the salaries, BA will likely as not try to avoid this option as long as possible - probably until someone designs a 99 seater which is approved to operate into LCY. Expect the 146/RJ to continue in service for a while yet.
* The one good thing to come out of the Open Skies debacle.