Sing a different tune (or song) change one's opinion about or attitude towards someone or something.
sing before breakfast, cry before night proverbial saying, early 17th century, warning against overconfidence in early happiness presaging a reversal of good fortune. The expression is recorded in French in the mid 16th century, and may also be found in reverse form, as ‘Cry before breakfast, sing before supper.’
sing from the same hymn sheet present a united front in public by not disagreeing with one another, especially when referring to a general policy.
See also little birds that can sing and won't sing, the opera isn't over till the fat lady sings.