Chiefweasel, that's incorrect. The core data is protected and cannot be modified. Using the editor, you can replace the spell in question, but if you remove the file with the replacement, the original will be in use again.
Although it would work to make a replacement for that spell, I recommend making a new copy of the spell, and not replacing it, because you might end up sharing data with someone else. If you do share data, and they've also made a replacement for that spell, HL will see that the same thing is being replaced twice, and throw up an error, since it doesn't know which replacement to use.
Instead, make a copy of that spell in the editor, remove all the other classes/domains that have access to the copy, and assign your copy only to the new domain.