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Phrase(s): change horses in midstream [and] change horses in the middle of the stream
Fig. to make major changes in an activity that has already begun; to choose someone or something else after it is too late. (Alludes to someone trying to move from one horse to another while crossing a stream.) • 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.
McGrawhill's American Idioms And Phrasal Verbs