The VHF-21D will directly replace the VHF-20B (i.e. no rack or connector change). However, the original controls for the VHF-20B (ex. 313N-5, CTL-20) are not capable of 8.33 kHz channeling. Therefore, you must replace the original control with the CTL-22C.

Please note that if the extended frequency range (116-151 MHz) offered by the VHF-20B is not required, then you can replace the VHF-20B with the VHF-21C instead.