There was one year that became especially proverbial for change.
In the year of the Seven Emperors. anno Septem Imperatorum...
...emperor #1 appointed his son joint-emperor. The senate already hated
the man and appointed an 83-year-old, who said he couldn't do it
without his son's help, so they were appointed the REAL two emperors:
but they died in a serious defeat. Two more tried. An unfortified town
wore the original Emperors down; starved their army; resisted a siege.
Emperor #7 was appointed in celebration. (Read the Loeb text here: posted by Thayer)
This is all too much for Vikipedia; who have one short page on another hectic year:
The Year of the Five Emperors, Annus Quinque Imperatorum which had much less change of course, but still slightly proverbial.
More simply, mutabilis, means 'changeable,' (describing a mind, a woman, a face, and (variusque labor) work.) . But if I met 'annus mutabilis,' I would think of normal change, new fashions, changing seasons, weather, the varying year. A stronger phrase would be : Annus mutabilitatis, a year of change. (ponounced mootabilita:tis).
If as a result of change everything is now upside down, back to front and inside out, say Annus Inversus.