You can teach an old dog new tricks! We would recommend adding some probiotics to the diet to help restore the good bacteria in the system. This will help to fight off any bad bacteria lurking around in there! This also helps to increase your veterans immunity. Yoghurts with live cultures are an excellent for this. Start slow, introduce new proteins every few days. Be sure to build up slowly at your dogs own pace. This will in turn give you both a chance to adjust.