Diarrhea from hydrolyzed cat food?

My 16 year old guy started vomiting every couple days, always after breakfast, so the vet recommended switching to hydrolyzed cat food. We tried a couple and he actually eats the Purina ProPlan HA, so we did a 1 week transition and he's been on it for 1 month now.

The problem is, his poop ​is getting more and more liquid, when it was solid on the old food. He's starting to get it on his legs and tracking it around the house because it's so bad. I know you're supposed to give it 2-3 months, but his vomiting has only slowed a little and the diarrhea is awful.

Can I just switch him back to his old food? Anyone else done this?