I had this same moment of panic when I booked at my 180-day mark, which was February 2nd for an August 2nd Boma breakfast reservation! In the rush of all things, it defaulted to today and clearly I couldn't make the reservation I had now made for 8 people (an "$80 value" of cancellation if you will). I panicked and changed the reservation for the right date, with no issues, all online.
To make sure, I waited an hour for the phone lines to open up and the lady who sounded like a fairy [chainsmoking] godmother on the other end told me that as long as I was able to change it online, I wouldn't be charged. While what she said was true, I don't think that was truly the mechanics of why I wasn't charged.