I don't know for sure what the Navy will end up with, but it appears to me that the 73 'could' replace the 9B, but will it?

Surface-search radars detect and track surface ships, boats, and low-flying aircraft from sea-level.

The AN/SPQ-9B is an X-Band, pulse Doppler, frequency agile radar, designed specifically for the littoral environment. It is part of the gun fire control system.

The AN/SPQ-9B scans out to the horizon and performs simultaneous and automatic air and surface target detection and tracking of low flying Anti-Ship Cruise Missiles (ASCMs), surface threats and low/slow flying aircraft, UAVs and helicopters.

The Future X-Band Radar (FXR) is intended to replace the aging AN/SPQ-9B radar. The FXR project was only started last year, so it will be a while before it arrives.

The AN/SPS-73(V)18 is an X-band radar using software-defined architecture to extend the radar’s range and navigation functions in bad weather; resist enemy electronic warfare attempts to jam it; detect UAVs, periscopes, floating debris, and floating mines; and improve collision avoidance in crowded waterways.
It simultaneously performs Navigation, Surface Search, and PDD missions. PDD is a high-stakes decision process where false target declarations are very costly and missed detection's are potentially fatal.