If I modify the game skills, they don't work. If I copy the game skill, rename it, make the changes I want, and then point to it, it works fine (with the triple-load trick, that is).
I'll try to be explicit (I don't know why so many people are so vague and coy about this). So, here's what I did to alter (technically replace) a skill within an existing mastery (as I did to change disarm traps):
1.) Import \records\skills\stealth\disarmtraps.dbr from database.
2.) Copy disarmtraps.dbr to same location (now named "copy of disarmtraps.dbr").
3.) Rename "Copy of disarmtraps.dbr" to "DRXdisarmtraps.dbr".
4.) Change whatever in DRXdisarmtraps.dbr.
5.) Import \records\skills\stealth\stealthskilltree.dbr
6.) Open stealthskilltree.dbr and go to the skill list. In there, skillName17 is "disarmtraps.dbr" which I change to "DRXdisarmtraps.dbr".
7.) Import \records\ingameui\playerskills\mastery 5\skill15.dbr (note that while skillname17 is disarm traps, it's skill 15 in the ui section--easy thing to confuse when working late at night. )
8.) Open skill15.dbr and go to config. Change the first field (skillname) to records\skills\stealth\DRXdisarmtraps.dbr.