Plan B is a secondary plan that you could fall back on in case Plan A, so to speak, didn't pan out as expected. The notion of "switching to Plan B" exists in English {Plan B}, French {le plan B}, German {Plan B}, Italian {piano B} etc.
What is an idiomatic way to express the idea of "switching to Plan B" in Russian? Is a single alphabet commonly used instead of an adjective like "запасной"?
запасной / запасный план
план «Б»
).