How do I transition my dog from home cooked food to raw frozen patties ?

I have heard that it is not good to feed cooked and raw foods at the same time because they digest differently, but i have also heard that it isnt good to switch to a new food abruptly. The only advice I can find online is how to switch a dog from kibble to raw but my situation is completely differen because i ahve been cooking my dog home made food for a long time. The raw food that I want to switch her too is Stella and Chewys frozen patties, mainly because I need her to lose weight (arthritis problems) and I like the variety of meats that they offer ( i cant afford to buy fresh lamb at 19.99 per lb., but would love to include lamb in her diet). Does anyone have experience switching from Home Made fresh cooked meat to raw frozen patties. And is this a good idea ?