JJJ, cats are very territorial and if the dogs new your cat will be REALLY put out. It's a good call to put the food upstairs. With the cat being old it may not feel able to take on the dog to show it who's boss, this is really important for a cat.
Where does your cat go upstairs? Cats always need a place where they feel safe, this might under a bed or pretty much anywhere, put its food and water there and make a fuss of the cat, let it know that its still loved and hope for the best. Cats are pretty adaptable but if the dogs new, rest assured the cat will be major league unhappy.
We had a cat that had nervous problems, he was overgrooming and making himself bald in places, The vet gave him the equivalent of prozac! It may sound bizarre but it worked wonders. Animals get mental problems as well as physical ones and at the moment your cat is probably feeling usurped, you'll have see what happens but definately ask your vet for advice.
Jerry
Live the life you love, love the life you live