Dog picks up a day early, is there a way to change the booking so they don't overpay?

I think if you contact Rover support, they can issue a refund to their account.

Rover Customer Support can process the refund. However, you will need to tell them the amount to be refunded. It should be in accordance with your posted cancellation policy, which is "moderate." According to that policy, the amount is also affected by how much notice they gave you about the early pickup.

• A full refund is available if you cancel by 12:00 pm the day before the stay begins. 12:00 pm is determined by the sitter's time zone.
• If cancelled later than 12:00pm the day before the stay begins, a 50%refund is available for the first seven days of the stay, and a 100% refund is available for any additional days.

While it may seem somewhat confusing, there is a reason, e.g., you may have been able to book another dog. If they let you know a few hours before picking up their dog, you should be giving only a 50% refund for that day. But it is up to you as to the amount.