Sea-change

Assuredly the fine old North Carolinan [William Alexander Graham], who has meanwhile himself gone under politically, as little anticipated while penning this sententious answer the "sea-change" as the sectional one which was to come after him. A "sea-change" has indeed transpired.

Source: wiktionary

Is it possible that our sense of humor has already undergone a sea-change? […] [T]he book as a whole seems strangely antiquated.

Source: wiktionary

There are other worlds than ours, and we never again return to the old place, because we have suffered not only a sea-change but a soul-change.

Source: wiktionary

Don't be swayed by minor setbacks: Don't confuse minor shifts with sea-changes. A bump in the road can—and should—be navigated without making major route changes.

Source: wiktionary