I hear folks say all the time that a Grey Market 20-30 year old Yamaha sounds just as good as a new one. I find that very hard to believe, seeing that technology is always moving forward, even with slight changes.
Technology does march forward, but technology is employed just as often (if not more often) to achieve cost savings as it is to achieve a superior product.
In the instance of Yamaha U series, you can look at a spec lisst and assume that all changes are mentioned, and that all metnionted changes are improvements, or you can play the pianos and decide on what has been achieved by the changes, both those changes cited as improvements and those not mentioned at all. And don't delude yourself into believing that piano makers cite every change made in the production process or the materials used.
I think that's the reason why Mike is suggesting that you play the pianos and I agree with him. As a player I feel instinctively from playing old and new U's that something has been gained by the technology and something lost as well. The result is a different piano.
I'd take a new U myself over an old for the obvious reason of its projected musical life, but I think it's naive to assume that every change Yamaha has made is all about producing a superior product.
If you're just looking for confirmation of your own belief that the current U design in its entirety must
be better than any older U design, then collect opinions. You'll get plenty of reinforcement for your belief from those who sell new.
If you want to put your belief to the test, do as Mike says. Play lots of U's....new ones and old ones in good condition.