Wow, cool. With that strategy, you never even have to buy oil.
I am going to be blunt for your own good now, kinda like my dad used to do.
Personally I think you are nuts. Trading every two years is going to cost you around 6-8 thousand dollars each time. What could possibly go wrong with the tractor that would even cost that much? I would think you could own this tractor for around 20 years and there still would be almost nothing needing fixing on it, judging by the 35 hours per year usage you are applying to it. In 20 years, if you trade every two years, you will have spent something like $60,000, plus the original $20K you have in your present tractor, plus increases in cost due to inflation over those years. I know, it sounds rediculous. Think of it this way: If you can somehow get your present tractor to last 6 years longer, it will be completly paid for, by what you would have spent trading. It still won't have 250 hours on it, so it may have its second oil change inside it yet. These direct injected compact Japanese diesels will run 4000-6000 hours most surely without problems, and possibly 10000 hours. Yours won't be broke in for several more years, at your current usage rate. Your tractor will outlive you if you change the oil every couple hundred hours. You'll need tires in about 20 years.
Hope this helps and makes you think. Spend your money on something that needs fixing, your JD is not broken.