Can this 200 amp ATS be used on a 125 amp service with out changing breaker to a125 amp

Hi, Gene. This is not recommended. The automatic transfer switch must be sized to match the rating of the service entrance equipment that it is connected to. Using a higher-rated automatic transfer switch on lower-rated service entrance equipment can result in various issues such as overloading the equipment, increased risk of electrical fires, and the potential for safety hazards. It's crucial to check with a licensed electrician to determine the appropriate size of the automatic transfer switch and all other electrical equipment when installing a home generator system.