The CPLD is not very likely to fail or lose its programming through a firmware update, even if interrupted. It's also not likely to be reprogrammed at all during a firmware update even though I would not rule that out completely. But you will not know until you diagnose the actual issue precisely.
IF the CPLD is actually programmed during a firmware update, then replacing it might be feasible. But you will need to do a little reverse-engineering of the schematic to find out: Get the CPLD datasheet, see where the programming pins are (this type of CPLD is programmed via JTAG, so you will need to find the pins labeled "TDI", "TDO", "TCK" and "TMS") and if they're connected to the actual processor. This will mean it's at least possible for the processor to reprogram the CPLD. It doesn't mean this is actually happening during a firmware update, though.