I don't think I would count Wilhelm as he remained King of Prussia and held the position of German Emperor in right of being king of Prussia.
In Alexander's case, it was the same kingdom, it just changed names.
Emperor D.Pedro I of Brazil abdicated his throne and became King D.Pedro IV of Portugal.
Leopold II was Grand Duke of Tuscany before becoming Emperor of Austria.
Maximilian I of Austria was not a king but a Viceroy of Lombardia and Veneto when he was invited to become Emperor of Mexico.
Joseph Bonaparte was King of Naples before moving to reign in Spain.
One might consider Vittorio-Emanuelle II who started as King of Sardinia and later became King of Italy.
Or Wilhelm I who started as King of Prussia and became Emperor of Germany.
Alexander I started as King of the Serbs, Croats and Slovenes before becoming King of Yougoslavia.
Monarch of one country and then another:
Henri III of France was King of Poland and Grand Duke of Lithuania from 1573-1575 before becoming King of France 1574-1589
Stanisław Leszczyński was King of Poland 1704-09, 1733-36. He became Duke of Lorraine 1737-1766
Franz Stephan Duke of Lorraine 1729-1737 became Grand Duke of Tuscany 1737-1765.
Carlo, Duke of Parma from 1731-1735 became King of Naples & Sicily from 1734-1759, and then King of Spain as Carlos III from 1759-1788.
Adolph, sovereign Duke of Nassau from 1839-1866 became Grand Duke of Luxembourg 1890-1905.
As Napoleon expanded his hold on Europe, several satellite sovereigns who possessed one throne landed on another. This included family members as well as dispossessed sovereigns who received other thrones as compensation.