I seldom transform from one feed to the other without an overlapping period in which I would mix the old with the new. I don't normally transition until six weeks of age anyway. It is hard to judge how much your chicks will consume and how long something will last. I never throw any feed away, I always have been able to mix it in. Patience is something I do not have, at least not much of it. But chickens has taught me to use some of it, patience, and let time expend any feed that is found to be around, at the time a new type is recommended. So a little every day will over time use it up, and mix it in without putting a spanner in the works.
The rules are, after all just guidelines and not the end of anything. Common sense, time and patience work hand in hand in bird keeping. Best to you and your birds,
RJ