I feel like I'm in crazy town. You absolutely SHOULD NOT tell the "original #1" program anything new. If you get ranked to match and they don't get you, then yeah they may be a little annoyed but it happens. In the unlikely scenario where you're hoping to go there for fellowship they may forget about this entirely or leadership in the program might even change. You don't get a gold star for "being honest," right now nothing good can come of telling them that you're not ranking them #1.
Whether you tell anything to the "new #1" program depends on how large your specialty is and how much sleep you're going to lose. If it's a small specialty where all the PDs talk, then you're stuck, you shouldn't say anything, end of conversation. If it's a larger specialty, my advice would be to still not say anything because it's unlikely going to make a difference. If you don't match there, it almost certainly will have nothing to do with whether you did or didn't send a love letter post-interview, plus the program has probably already made their rank list at this point. But if this is going to keep you up at night worrying for the next month if you don't send the email, then it's also not the end of the world if you send them a message.
For future applicants, this is precisely why if you choose to send #1 love letters that you shouldn't do it until you actually have your ROL in order.