Brightonian means born and bred
you might have been born in Brighton and lived somewhere else all your life but you are still a Brightonian and the reverse is the same you might have lived in Brighton nearly all your life but it does not make you a Brightonian.
This with bells on
I haven't lived in Brighton since 1998 but I was born in Central Brighton and grew up there and still call myself a Brightonian.
I hear people being called Brightonians just because they have lived there for a couple of years.
A Scouser living elsewhere other than Liverpool is still a Scouser.