If he's still going to get hay (which is good because he needs the long fibers and chew time) than simply replace senior for what calories he isn't getting out of the hay. Start him out building up to 5# per day. Keep adding until you get him up to the condition he needs to be at. Could easily be 10#/day. BP, nutritionally, is pretty close to grass hay. It is extreely low in fat (1%) whereas grass hay is about 3%. My big knock against BP is you pay too much for what you get out of it. As long as he can still chew hay, save yourself the $$ and just go with the hay. If you want to add fat, add oil directly to the senior. They're cheap calories that are easy to get into them. A C of oil will have 2Mcals at a cost of $.20. Those same calories in the senior will cost you $.54 ($18 bag).