Hi Leighann,
You can't edit the stay total once its's been booked and paid, but you can edit the total beforehand. There isn't a spot to specify the price for existing customers, but what I've found in the last two and a half years is that a verbal confirmation will do the trick!
If I had raised my price, and received a request from a past client, I would reply back something like 'Of course, we'd love to watch _ again! - I slightly raised my prices but will edit the total to honor your original price per night.'
Sometimes people like to confirm each and every time that I will discount the stay, that's fine - but it's also gotten to the point with others that they just know I will edit the total before I accept the booking. Hope this helps! Let me know if you have any other questions, I have a lot of experience with this.