The phrase "existing reservations" refers to plans or bookings that have already been made for something, such as a hotel room, flight, or event. It means that these reservations are already confirmed and in place. Full definition
This offer may not be combined with other select promotions, offers or discounts and is not valid for existing reservations or groups. (waldorfastoriaorlando.com)
Provide support with hotel inquiries, drive revenue, and modify / cancel existing reservations. (flexjobs.com)
Offer not valid on existing reservations and may not be combined with any other offer. (southwestvacations.com)