OK, I just spent a couple hours testing the OTS change function on the SX-900.
Based on great suggestions by the group....I can not make it fail....let me explain.
I am using the Yamaha playlist. I am using only Yamaha registrations called by the playlist. I have not changed anything.

So, Yamaha styles right out of the box. OTS link ON.

IF I do as suggested and be sure to lift my right hand before changing OTS they all work fine, no bad sounds no problems at all. I have done it very fast as well and still is fine. Even with voices that decay pianos, guitars etc...as soon as I lift my hand and hit a different OTS all is good. The previous sound STOPS as I would like and I can immediately use the next voice.

So maybe they fixed this issue? As soon as I lift my RH, and immediately change OTS the sound does STOP immediately....Yeah. I can see where if you hold the RH notes down it can and will cause the bad noises, abrupt change etc....after all you are telling the instrument you want to play the old sound still and switching while it is playing.

A nice menu option might be to kill all notes when you change OTS (just like it does when you lift your hand)...then you could immediately play the next notes that way you could hold notes into the next measure but change the Variation at the measure line.....but I will just use a fill or break instead of changing OTS in the case where a not ties into the next measure.....

And the sounds on the 900 are really great! I do a lot of orchestral stuff and it sure sounds good...

Thanks for the help everyone!