Integrated communications systems having a scalable or upgradable TDM switch fabric (i.e., e.g., TDM-controlling switch/MUX) are disclosed. At a first point in time a system is first sold, installed and utilized with a first TDM capacity, using a first TDM switch/MUX controlling a first set of TDM streams operating at a first frequency. A first set of line and other cards (e.g., DSP resources) are provided to provide or receive the first set of TDM streams. At a second point in time the system is upgraded by installation of a second TDM switch/MUX; the second TDM switch MUX controls the first set of TDM streams operating at the first frequency and also controls a second set of TDM streams operating at second frequency, which is a frequency different and preferably higher as compared to the first frequency. With at least some of the first cards coupled to the TDM bus, the second TDM switch/MUX couples TDM streams to and from the first cards using the first streams at the first frequency, while concurrently coupling TDM streams to and from the second cards using the second streams at the second frequency. The first switch/MUX preferably operates concurrently with the second switch/MUX to couple streams to and from the TDM bus (e.g., from an HDLC or multi-protocol framing engine, etc.), while the first switch/MUX does not operate to control the TDM bus, as this function is carried out by the second switch/MUX.