To Edit/Change/Replace a Factory Style in any Style Set, you must first go to Global>Mode Preferences>Media and 'un-check' the Factory Style and Pad Protect. You can then do whatever you want with the Factory Styles.
When I want to add or replace a Factory Style, I usually D/L the new style into a User area and then 'Copy and Paste' the style from User to Factory.
Always back up what you do, but I do not think these settings will change unless you change them or do a Factory Reset.
However, keep in mind that if you Edit the 'Bossa Nova' style in "Latin P1 Slot number 3" and set it up for a song, let's say "Girl From Ipanema' and put it in a playlist, if you later change "Latin P1 Slot number 3" to "Classic Cha Cha" and move "Bossa Nova" to another location, you must then re-configure "Girl From Ipanema" to look where "Bossa Nova" now resides. The notation "Girl From Ipanema" in the 'Songlist' (Playlist) refers to the the LOCATION of the associated style, not to the style itself.