
I would guess he is on any brand of tinned food - as the charity usually relies on donated tinned food.
When I worked for the shame charity we had an Arden Grange Rep come and give a talk on the food - I seem to them remember saying that within the AG range it was fine to just change the foods over - I have done this many times with my own dogs between adult/premium/light and senior all without ill effects. Although If you are worried then changing him over a few days should do any harm and would probably be advisable as hes changing home as well.
Compared to the Adult the senior will have lower protein and usually oil and fats - although higher in other areas. If you take a look at the AG website you will be able to compare to different foods.