I don't know that you'll get a real specific answer to that question @MajorDan - it's more about timing than it is about temperature. You know winter is coming by the calendar - go ahead and switch if you want.
And remember we're talking water temperature, not air temperature. Define "getting cold" - it's 37 degrees here this morning!
Hi. I would suggest switching to fall/spring diet when the water temperature starts to go down to the low 60’s. Below 50-55 degree range I would stop feeding altogether. Also I would recommend to reduce the frequency of feeding also as temps go down so if you are feeding them two to three times a day in the summer then in the fall go down to once a day. If you feed them once a day in the summer then go down to every other day or just a few times a week. They need a lot less food as the water gets colder. Once you take them off food with protein they are not growing so much as storing fat to help them get through the winter.
So our temps dipped here. It went from 72 degrees to 52 degrees in one day. I finally “braved” the weather this afternoon to see how my fish were doing. They were all lower down in the water and definitely not at the surface asking for food. No need to overthink the situation. It was time for me to stop feeding until things warm up!