To use ASIC would require a huge volume of missiles built just in order to fab for the customized chips for the application. If you use something generic but programmable, like an FPGA, you can build for a far smaller volume of missiles. Using an FPGA lets you correct bugs in the system using updates.I don’t think it is as simple as just a software patch.
1. We don’t even know if the guidance system is programmable or if it’s all ASIC.
2. The proximity fuse is ill suited against surface ships. The frag warhead can damage electronics and kill sailors on deck, but will not be as damaging as a direct impact weapon. For that they need a contact fuse. Therefore, a hw change.
3. The radar on the missile needs to support surface attack mode. A near stationary target will have different radar echo properties than a moving target (think of doppler). This goes back to 1. I suspect also a radar circuitry upgrade would be necessary since at least some of the processing would be analog.
#2 is sufficient to mission kill a ship if the sensors are knocked out by HE-FRAG.
#3 yes, you should be looking at changes in the amplitude and the doppler.