I'm already baking a cherry pie. I can't bake an apple pie. It's too late to change horses in the middle of the stream.
•The house is half-built. It's too late to hire a different architect. You can't change horses in midstream.
•Jane: I've written a rough draft of my research paper, but the topic doesn't interest me as much as I thought. Maybe I ought to pick a different one. Jill: Don't change horses in midstream.