Well, it's never that simple is it? The regular non-fat AMIC is a 160 pin chip and the FAT-AMIC is a 208 pin chip. If it was a simple chip replacement and socket installation I would have already done it.
The regular AMIC is the one found in 7100 and 8100s which only supports three NuBus slots.
In the Power 80/100/120 the AMIC chip resides on the I/O daughter card, which is unique to those machines. To save room on the circuit board, PCC put one of the SCSI ports and (IIRC) the serial ports and the DRAM video out on a daughter card, which contains that circuitry. I may be misremembering some of that stuff.
Anyway, to replace the AMIC with a FAT AMIC you'd pretty much have to redesign that I/O card to take the 208 pin FAT-AMIC and it's a very dense card, full of components. Then move the components over from an existing I/O card (found some extras at Goodwill years ago). I expect that the needed connections from the additional two NuBus slots do travel to that I/O slot's connector.
Having examined the issue in detail, it is my belief that PCC was pretty clever. Their original plan was probably to sell a three slot and a five slot version. The three slot version would have two unpopulated NuBus connectors, and use the AMIC version of the I/O card. The 5 slot version would have all five NuBus slots and come with an I/O card bearing the Fat AMIC. My understanding was that they couldn't get Apple to go along with the five slot version, or maybe they just couldn't agree of the price.
So the only difference between PCC's three slot and five slot versions would be the I/O card. Everything else would be the same component. That is good cost control.
So, the question is, was there ever a five-slot I/O card made? If so, where is it now?