For a senior cat, the use of nutrients and organ functions will collapse gradually and acceptance of excess or insufficient nutrients will be not better. Requirement of calorific capacity will reduce gradually because of decreased activity.
 
Choosing proper foods contain proper protein which can be used completely by your senior cat should be noticed. The decrease of the phosphorus and sodium uptake also should be noticed at the same time.
 
If your cat is very old, please don't try to make him total vegan. Some old cats take to it easily, but some can be very set in their ways and it would be cruel to force change on them at this time of life.
 
You could slowly reduce the amount of meat or chicken feeding him mainly vegan food and only a very small amount of meat or chicken. Please if you must give meat, fish or chicken make sure it is fit for human consumption.