"Nicolas Sarkozy : «Nous sommes la seule alternance crédible
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10 déc. 2015 — INTERVIEW - L'ancien chef de l'État estime que voter pour les Républicains dimanche permettra de «sanctionner» les socialistes sans ..."
If you're talking binaries -- left/right -- "alternance" is fine. But what if, in the example above, power went from the Socialists, to some other, imaginary, party on the left, that's not "alternance", left to right, that's an alternative.
The words are available in French to say something like "une transition pacifique de pouvoir".
AC/DC -- alternating, binary.
Conclusion: "something else", change/changeover (see Lisa's answer) is good, I think, alternative.