Yeah, that's a big enough upgrade that it'll take up both of what you were getting. The other veterans that are left will get two abilities from this piece, while newcomers all get three abilities.

EDIT: Last thing on the bosses topic. If you're running a world and someone comes and yanks the rug out from under you with a one-shot kill to a boss, you are free to do a workaround in a similar manner to what I did here. I am not exaggerating when I say outright annihilating bosses will not be tolerated.