As an owner of Old horses, 30+ yrs old.. it is more than just feed . Perhaps you could ask where you currently board , if you could get a discount on the old horse, if you supplied ALL of its feed. Or maybe have him in a corral with a cover and not a box stall, or put the "new" horse in the a pen with a cover and not a box stall.
Sr horses, have more needs than just extra feed, small issues can turn into huge Vet bills.
I would Never send one of my senior horses to be used as a kids horse, or as a pasture pet to someone else.
I DO WITHOUT so my horse don't have to . Cut down the clothes bill, the going out bills, eat cheaper .
I will go to second hand stores if needed, to make sure my OLD horses Have what they need.