Hi Al,
fully customizable ( if there is such a word haahaa).
I wouldn't rush into replacing styles straight away as there are a heap of empty slots available for user styles.
In the long term though, you can set up the keyboard for your own needs.
Default on the keyboard, locks the user styles in so that you don't accidently overwrite them, the lock can be taken off so that you can.
One of the most important things to learn is how to save & load your user data to usb stick &/or your PC.
The loading & saving is different on the korgs to what it is on the Yamaha's.
Check out the Multimedia section of your manual. If you've got your user data saved correctly, then you won't have hassles should you accidently delete important user files.

best wishes
Rikki