Easy way to wean cat off of Fancy Feast and onto Hills Prescription Diet?

I'm trying to get my elderly mom to stop giving her cat (with a granuloma
like skin condition on her hind legs) Fancy Feast and try Hills low
allergen Z/D prescription diet formula to see if it was the Fancy Feast
that's the problem.

This is causing quite a stir. My mom has mood swings (from her anti-
anxiety and anti-depressant meds) and the minute the cat cries for more
food, she wants to feed it Fancy Feast or the low sodium turkey breast
she was giving it with her Triamcinolone. When I tell her that she can't
feed her that stuff for at least 6 weeks, she cries more than the cat
does and starts calling me non-stop and yells all kinds of nonsense over
the phone.

We both love the cat and I thought that by getting her this beautiful
animal, she might calm down. When the cat developed the skin condition,
it became too much for my mom to handle as she has phobias about all
kinds of doctors, and it really creates a storm when any kind of test is
needed.

I don't know if it's her age or the meds, but she seems incapable of
understanding the need for the food change. Last night she called me
crying because she felt the Hills z/d dry food is too large and too hard
for the cat to chew (it's probably the same size as her previous Purina
Pro Plan), and I should bring it back! Then she asked why I bought such
a big can as she likes small cans. I told her that the cans only come in
one size. That also caused much aggravation. Over the next hour there
were about 8 more heart palpitating calls with me trying to explain the
skin situation over and over. She's never been easy to deal with and she
seems to have gotten worse over the past few years.

Today, I've gotten a few calls that the cat is crying for more food.
I keep telling her to just pet the cat and eventually the cat will adjust
to this food.

Any suggestions? The cat has become very attached to her and loves
sitting on her lap for much of the day and sleeps with her.

PS - I've told her it might be a carpet mite allergy as she's never steam
cleaned the carpet in the bedroom the cat loves to sleep in. Don't ask
what went on after that suggestion! Her response was that there's no
mites in her carpet and it doesn't need to be cleaned, only vacuumed
(it's over 20 years old).

I'm close to giving up on the pill and food treatments and just doing
what others here have done - go the Depo Medrol route with intermittent
injections (even with that my mom found a problem. When we gave her two
injections about two years ago, the cat was lethargic for a few days and
that led to more anxious calls). I was thinking of going to a vet
dermatologist, but between the 40 minute car ride, and the eventual cost
of the bill, I don't think my digestive track could take the resulting
harangue from my mom! Secondly, most here haven't had luck with the
numerous tests performed on cats with skin conditions.

Easy way to wean cat off of Fancy Feast and onto Hills Prescription Diet? - HELP!