You can, in fact Yamaha encouraged that in previous (Ty2) manuals.
Copy the styles you're interested in - maybe all if you wish - to your own folders on the hard disk or USB (or User Drive if not a huge number and there is space).
Then set up the relevant style buttons to access your folders containing the style copies instead of those in ROM.
Since they are in RAM you can now modify the styles and resave at leisure (to the same names or different depending on preference).

john