Typically Kubota has adjusted their base prices on March 1, at least in Canada. Usually their prices go up about 7-10% per year.
The only real trouble you may encounter is being unable to find an "old stock" 3940. Often a dealer won't have the machine you want on the lot, but will be able to find one at another dealer and ship it in. In the unlikely event a 3940 simply isn't available until after March 1, you MIGHT get stuck paying the higher price if there is an increase. That's when you haggle an available 4240 down to the same price. Dealers really hate to see customers walking out the door without a signed purchase agreement, particularly when the deal fell through because what they wanted to buy wasn't available. Once you're out the door, New Holland and Deere are just down the road.
I had something similar happen to me. When we negotiated the price, it was based on a pre-March delivery L3400 being available locally. When I went in to finalize the deal and order one, the closest one they could find was in Quebec. This was in October, getting close to the end of the model year, not that it makes a huge deal when it comes to tractors. If it had been the end of the production run or if all they could find was one that had been sitting on the lot for 2 years, I might not have an L3400. As it was it turned out well, they found an 09 that was pre-March delivery, so the original price I had negotiated applied without any argument.
Incidentally, don't let them BS you about tire types. If you want R4's, they should be the same price as R1's. I was told the original tires were the same cost regardless of which type you wanted. R4's are a more expensive tire, but the rims are cheaper so it balances out. So, if they say "We found a 3940, but it has R1's..." the correct response should be, "Well, I guess you're going to have to swap them, aren't you.." Maybe it was just my dealer that was so accomodating, but I got the impression it was a Kubota policy.
It's an expensive proposition to swap tires after the sale.. plus you're stuck with the old tires and rims. Even if someone here wanted to buy 'em, shipping is a killer.
Sean