I suspect that some computer hacker will find a way to crack Deere's code. Although Deere might make some progress stopping a few attempts to hack their stuff, they will find that 10 other people will continue hacking Deere equipment.
Although it may be illegal, I'm certain there isn't going to be a successful crackdown. It would be about as effective as people download MP3s and using emulators with ROMs instead of buying console games.
The other reason there may not be a crackdown is because there would be a way to embarrass politicians who wish to enforce Deere's proprietary code. The the farmers and low ranking mechanics will claim that they are the poor people being crushed under a huge corporation, known as Deere.
The other problem Deere would face is that companies that make equipment simpler to service will gain in popularity. Just think of the people who had a VW, couldn't stand the high repair and maintenance costs, and bought a Honda or a Toyota.