I can see what you mean.
The same message appears to me when i try to edit a preset part,except from RHY1+RHY2.
""You have to delete the data of the original style before recording.Delete?""
So the parts that give that message I cannot make any change to them,
except from using the mixing console to change their voice,volume,brightness etc
or use them to another style with assembling.
I believe that if it is a part that recorded by you or any other user that you have downloaded the style from and he recorded that part,
only then you can edit the part,
or record extra notes to that part if you wish, without having to delete the original notes if you don't want to.
I guess there are data on preset Yamaha style parts that cannot be changed by users.