What code are you rebuilding?
If this is a core Unreal Engine class, you will need to re-generate and re-build both the editor, and the support code for packaged games, and then make sure you start the new editor you built, not the one from the Epic Launcher.
It might be better to create a DamageType subclass that has the new fields you want, which can be done in a C++ plugin in the regular editor.
You would then make all your damage types you actually use derive from YourDamageType, and when you get a DamageType in your game, cast to YourDamageType to read the values.