As noted in a previous post, Kubota usually changes the numbers when the tractor changes. Most times, it is as BP said, an evolutionary or relatively minor change. For instance, the only significant difference in the BX 22 and BX 23 is that the loader valve stays on the tractor when the loader is removed from the BX 23. The valve came off with the loader on the BX22 making it duifficult to use with other attachments.
I would have to say the differences in the BX23 and the BX24 are "revolutionary", to borrow BP Fick's words again. Almost no parts are the same, including the body work, frame, transaxle, front axle, engine, brakes, front wheels, 3 point hitch, instrumentation, electrical system, hydraulics, loader or backhoe. The BX24 was essentially a totally new tractor.
In the number of changes made, the BX25 was again, an evolutionary change with return to metal fenders and hood (engine side cowls remain plastic), and reduction of the abrupt stop in reverse that seriously annoyed some owners. Almost everything else remained the same as the BX24, including engine hp (and the exception of price of course).
All of the tractors are excellent reliable machines with real capabilities. Obviously, each has a well deserved, extremely loyal following. You will enjoy whichever version you finally choose.