I did something similar on my old Sanyo EJC-B355. Fortunately, there was a cavity at the back of the case big enough to hold a pair of AA batteries in a holder. I clipped the leads to isolate the onboard coin battery and soldered on leads to the batteries in the holder. That was quite a few year ago and it's still working.

Unfortunately, the cooker has now started to leak somehow, so water collects around the rim of the pot and sometimes runs down to the counter. so now looking into a new cooker. But as far as I can tell they ALL have that onboard battery. How do they get away with that? It's like buying a new Chevy and having to send it to the crushers when the tires wore out!