I can only speak about Pacific Bell (Pacific Telephone) in the specifics of embedding newer equipment a given C.O. Pacific Telephone, unlike its eastern and midwestern cousins, had steppers en mass in urban C.Os.
No. 5 XBARS were installed to serve one office code within those C.O.s as soon as the No. 5 XBAR became available. The 5 XBAR would serve only one office code (prefix) but it served as the toll machine for all the other "dumb" SxS machines.