We actually have a constant in our codebase to mark such bugs. You found one? Put it inside an:

    if (PREFER_EXISTING_BUGS_TO_NEW_ONES) { theOldBehavior(); }
    else { theNewBehavior(); }

Then, if we ever go to do a v2 of the thing, we can review these existing "load bearing bugs" and see if they make sense to change.