Frank Sinatra was replaced in about 1951

Did you know there were two Frank Sinatras?

The first Frank had a completely different voice from the second Frank. He was replaced in about 1951.

In Double Dynamite (1951) (see [link to en.wikipedia.org] you can see both Franks. The dancing Frank is the original, the replacement Frank does some acting. The film was originally called "It's Only Money" and took 3 years to make - started filming with Frank 1, ended filming with Frank 2. They also both look quite different.

Ostensibly the name was changed to Double Dynamite as reference to Jane Russell's famous assets; in reality it was changed as a code that Frank had been replaced by a DOUBLE.

The Frank of the 30s and 40s sounds alot like Bing Crosby, both sings and dances and is in a number of light-hearted musicals.

The Frank of the 50s onwards has a completely different and quite distinct voice, arguably a much better one but nothing like the original. We no longer see him dancing AT ALL but now he suddenly is into hard-broiled acting starting with "From Here To Eternity."