I think we had crossed lines, because I was thinking that you can play the styles from the earlier models, but it never occurred to me you were talking about actually using - say a KN5000 to play the styles from the KN7000 because of course, the OS will have changed so even if it were possible, it wouldn't sound the same because it would be picking up the KN5 on-board styles which as you say, would sometimes produce some 'weird' sounds to say the least.
This is what I meant when I said I was under the impression you could play the T3 on the T4. If the T4 on-board styles match those of the T3, then of course the only difference will be the actual 'sound' which is slightly better than the T3's.
However, if the T4 cannot match the style, it'll find the nearest on- board one and then, what you hear isn't always what you're expecting. In fact, in some cases they are more than 'weird' which means you're once again back to all the button pushing in order to get a decent set.
It's all the button pushing I hate, it is so time consuming. This is an area that Yamaha should take a long hard look at, I think.