Kubota does what nearly every tractor manufacture does. Take the same engine, transmission and change the hp rating. In the small tractors it is usually the addition of a turbo or intercooler to get the different hp rating. In the mid and larger ag tractors, often the only difference is the computer module but the hp difference may 50-75 hp. Usually the injectors and fuel pumps are the same. These tractors will have the same part number for pistons, injectors, fuel pumps etc. The computer regulates the amount of fuel injected. The computers can be reprogrammed, but probably no dealer would do that. The manufactures want the exact serial number before any significant programming could be done to their chip. There are several aftermarket companies that have chips that can be used to change hp in the mid and large ag tractors by 10-35% depending on the desired setting and cost in the $500 range. Many of these devices plug right into the wiring harness. I don't know this for fact, but I believe they fool the computer into changing the hp by altering the fuel rail pressure reading. The computer reads a lower pressure reading and thus increases the pressure in the high pressure common fuel rail systems- which is basically what the manufactures does.
We have a Case-IH Magnum 245 (245hp). The tractor is identical to their 215 (215hp) and 275 (275hp) with the exception of a few features that are optional on the smaller sizes but standard on the large size. However, there are tens of thousands of dollars difference in the price. Dealers can discount the bigger tractor more than the same size smaller hp tractor due to the significant manufacture mark up on the large hp tractor, which is nearly the same tractor.