You mustn't be concerned about changing your canine's diet from adult meals to Feeding Older Canines, if the dog is healthy, medically fit and keeps a stable unwanted fat. On the contrary, if your dog's weight becomes unstable or digestion issues arise, then you might have to interchange. But if weight is truly the only problem, try limiting how much dog food you feed to doggy. This could be useful in addressing the actual load problem.
Rescue dogs should always be spayed or neutered, vaccinated, and the equivalent of relatively healthy before purchase. A dog on medication food for an elderly dog an ear infection or arthritis can easily go to a different home