Driven by the future need of exploiting the so-called extremely and tremendously high-frequency bands for the rate enhancement of microwave cellular networks, in this paper, we consider the possibility of upgrading the already existing infrastructure of microwave base stations (BSs) by enabling them to convey information through microwave or terahertz (THz) links, depending on the distance and whether a line-of-sight (LOS) link exists between a mobile terminal (MT) and its serving BS. By using a tractable stochastic-geometry approach for the modeling of the position and number of BSs and possible blockages in the radio path, we derive analytical formulas for the achieved coverage probability and average rate, which exhibit a close match with the simulation results and validate the expected significant average-rate gains with respect to the communication only through microwave links. In addition, the results show that the expected tradeoff between the average-rate enhancement and coverage-probability reduction due to the high path-loss attenuation of the THz links can be greatly counteracted by employing highly directional antennas in the THz case, especially for very dense BS setups. © 1967-2012 IEEE.