When did Thessalonica become Thessaloniki?

It is the same word Salonica or Thessaloniki or Thessalonica. You choose!

When I was there in '69, the locals called it Saloniki, though it was Thessalonica on our English map.

Thessaloniki is the name of Aexander's the Great sister and the city was called in her favor. Over the years many cultures lived in the area like the Hebrews from Spain and the Slavs. Remember that Thessaloniki was occupied for almost 500 years by the Turks. The names used during the period are Solun, Salonica, Saloniki. After the city's libetration in 1912 the official name was brought in use and the majority refers to that. Some locals still call Saloniki their city.