No you cannot change the extra guest fee for seaonality. You need to reprice your listing for high season to cover the fee you want, whether there is 1 guest or 6. The listing is based on the place, not on the guest numbers. The extra guest fee should cover the costs of the extra linen, beds you need to service, breakfast or other wear and tear after the FIRST guest. So if 1 person rents, what is the least you will take for your property. If there are 10, they should just be covering the extra work, not contributing to the base price. Think of the extra people as additional revenue, not a part of the base price.