Funny story. They say u r supposed to slowly mix the new food in with the old, increasing the new food amount a little at a time.
You can. Its probably a good idea if you are switching from a really poor dog food to one of the high quality kibbles as they generally are very high protein. The difference in the protein content can be staggering. I would keep an eye on their stools for awhile when I switched foods. Consistently runny stools would be the first sign that food was not going to work.
BTW my dog LOVED canned tripe. Would go wild for that stuff.