You clearly have been doing your reading if you know about FLUTD, and specifically, idiophatic cystitis, which is a component of that condition.
Here's the problem: Even if improved, for a female cat, it is critical to get the urine tested. Why?
Well, she was also vomiting. That can be very common with urinary tract infections.
So, I would strongly advise against a food change until that urine is tested, or the diet change could possibly make Cleo worse.
Now, regarding food changes:
FLUTD and cystitis is believed to be stress-induced. When it happens once, it can happen again. One of the most popular foods, I believe in the world, is the prescription diet Science Diet C/D multicare stress formula. It is clinically proven to reduce the incidence of FLUTD.
Yes, as you read, it is believed that cats with bladder issues (or also with chronic gastrointestinal problems) may benefit from an all canned diet.
I have no reason to believe that the Merrick will be o any additional benefit. Also, there is no research to support a grain-free diet is any better. It is true that some pets can have allergies to grains. But!
MOST food allergies are usually due to the protein in the food not the grain.
Changing to Merrick and hoping the issue will resolve won't work. If the issue resolves it is simply because the cystitis (bladder wall inflammation) resolved on its own.
Diet change will almost never remedy a acute onset of cystitis. Food change is more for helping to prevent recurrence.
Well worth the urine test as a precaution.
Hope that information helps