What happens if you change dates on a prepaid rate?

I had booked a stay at a HP during the 30% off promo and later GP was kind enough to let me change the dates of the stay. The rate changed to the refundable one, but I'm happy to pay the difference for the convenience. I was wondering if you can help me with the following point. Is the prepaid rate going to function as credit for the refundable rate? I hope I did not lose that money. And a Costco cert should work on the refundable rate, right?

To add to my confusion, I was going over my CC statement and I'm not sure if I have been charged yet. I have a bunch of prepaid stays with that property and not all have been charged, but I can't figure out how to connect charges to dates. Maybe they let me change the dates because I hadn't been charged yet?