Introducing new foods or switching your cat’s food all at once can cause a series of gastrointestinal problems like vomiting, diarrhoea and a decreased appetite. If you are planning on changing their diet, the transition that you make to the new diet should be very gradual so that your cat’s system gets enough time to get used to the change without causing any distress. Ideally, these transitions should happen over 7 to 8 days. During this transition period, you should gradually incorporate the new food by mixing it with your cat’s present diet. Here’s how you should do it: 

1. Day 1 and 2: Mix 20% of the new diet with 80% of the old diet.  
2. Day 3-5: Add 50% of the new diet with 50% of the old diet. 
3. Day 6 & 7: Add 75% of the new diet with 25% of the old diet. 
4. Day 8: 100% new diet.

The above-given switch works for most pets. In cases of cats with sensitive stomachs, this transition can take even longer. The key to an effective diet transition is to keep an eye on your cat’s response. If at any point during the transition your cat displays concerning signs like diarrhoea, vomiting or changes in his/her appetite, it may mean that you should proceed more slowly. And if you think that the transition has been gradual but your cat is still showing signs of discomfort, then it is best to consult your veterinarian about the same.