LO is coming to 13months and is taking breastmilk 3 times day. I'd like to start introducing formula milk but which feed should I replace with for a start? Morning, afternoon or evening?

I think the below table might able to help you. That how I convert my lo too. https://media.giphy.com/media/3o6ZtqcttXaTJi7dYY/giphy.gif

I agree with Ling. Try morning feeds. If there is any reaction you will be able to see and notice it too

maybe you wanna try the morning feeds so if he reacts to the milk at least it's during the day.