The older things get, the more remarkable it becomes when they've managed to stay unspoiled. I'd agree that 20 years ago, a 78 P Bass wouldn't be held in much regard, but that was 20 years ago. Now it's nearly 50 years old and still has all it's original parts. Regardless of if it's a gem or not, it still seems a little sad to start pulling parts off of it now after it's stayed intact for so long. Changing the pickguard is especially egregious to me. There are plenty of brand new P Basses that can be done to.
Ultimately, it's the OP's bass and he can do whatever he wants, but it'll just be one less instrument that stayed together and that's kinda sad to me.