Weaning Kitten Onto Gruel Caused Diarrhea

Just yesterday I went to the shelter I foster out of to see the vet about my kitten who had severe diarrhea and weight loss. She was originally aged at 2 weeks old, but I quickly discovered she was a very tiny 3.5+ week old. She suddenly stared refusing formula and we were forced to make gruel out of adult cat food. She had massive amounts of diarrhea and some weight loss. The vets looked at her, ran every test they had, and concluded the culprit was the sudden change in food, sending us home with some probiotics and kitten food. She is still having some diarrhea and a small amount of weight loss (though she is very active and has quite a voice). My plan is to start feeding her every two hours until we get consistent weight gain, but I am wondering of I should convert her back to formula, or would that make the situation worse. Any advice on what I can do for this baby would be greatly appreciated!!