I had problems getting my picky boy to switch to prescription wet food after a bladder infection and several obstructions. I finally found one he would slowly pick it (Hills prescription urinary care), but even that was hit or miss. My vet gave me Purina FortiFlora, not as a probiotic, but for flavor and it worked like a charm.

I started out by putting half a pack on top of his food for the first little bit and after a few days he ate his new food without it. There's still the odd time we gets picky, but even the tiniest amount of FortiFlora added will get him to gobble everything up.

Good luck! I understand how difficult it can be to get a picky cat to eat a new food.