I have not actually programmed a Fanuc 90 series, but I do have some reference manuals, and they mention that two options are present, Ram memory with battery back-up, and EEprom, which is a permanent form of non-volatile memory.'
With the PLC's I am familiar with, the battery & memory is in the CPU unit, so there is a good chance replacing the Power supply will not affect the memory, if it is EEprom memory then you don't have anything to worry about, anyway.
There should be a small access door on one of the units, probably the CPU, where the battery would be, if fitted
If I get a chance later today, I will dig a bit deeper in the manual.
Al.