7
While not precisely the same case, I believe this is included in this halacha (O.C. 268:6):
הטועה בתפל' שבת והחליף של זו בזו אינו חוזר
One who makes a mistake in the Shabbos prayers and switches one for the other (e.g. shacharis for mincha), he does not go back.
The Mishna Brura there (14) explains:
(יד) אינו חוזר - שעיקר ברכה רביעית היא רצה ...
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