Confirm it is the recipe, not your cat
Manufacturers reformulate for many reasons — ingredient costs, supply, updated nutrition standards — and the change is often announced only by small new-formula wording on the packaging or a quietly altered ingredient list. Compare an old bag or can, if you still have one, against the new stock. At the same time, rule out the other suspect: a cat who refuses the changed recipe but also eats less of everything, hides, or loses weight may be unwell, and dental pain in particular can look exactly like pickiness. If her appetite is down across the board, the vet comes before the food hunt.
Buy yourself a bridge with old stock
If the recipe change is confirmed, search for remaining old-formula stock — different shops, the manufacturer's own outlet, and the sizes or formats your usual shop does not carry. Even a few weeks' supply transforms the problem: instead of a cold swap tonight, you have a bridge across which to walk her gradually. Check dates on what you find and store it sealed and cool, and be honest about how long the bridge is, because rationing the old food into ever-smaller portions without a destination food only postpones the same crisis.
Choose the closest alternative, not the most advertised
Your target is the food most similar to what she already accepts: same main protein, same texture — pâté drinkers rarely convert to chunks in gravy overnight — and a comparable format and smell. Read ingredient lists against the old recipe rather than front-of-pack claims. Buy single cans or the smallest bags of two or three close candidates, because a one-food cat's owner quickly accumulates a cupboard of rejected full-size purchases. Similarity gives each candidate its best chance; novelty is exactly what she has been refusing all her life.
Blend across the bridge in stages
Serve the candidate mixed into the old food in small proportions, holding each stage for a few days and increasing only while she eats willingly. Warm the mixture slightly to lift its aroma, use her usual bowl in her usual quiet place, and put the meal down at her normal times so nothing else about dinner changes. Keep a simple log of candidate, proportion and outcome — cleaned bowl, partial, refused — so patterns emerge: she may accept any fish pâté but reject poultry in every form, which narrows the search faster than memory does.
If every candidate fails
Do not let loyalty to the search leave her hungry; a cat who will not eat needs her vet involved, both to exclude a medical cause and to plan a safe route to adequate nutrition. Bring your log to the appointment, because a written record of what was offered and refused saves repeating failed trials. Going forward, once she settles on a new food, quietly rotate in a second accepted food while times are good, so the next reformulation is an inconvenience rather than an emergency, and put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.