Recently he sent me this message:
I have been playing the tune Six Feet Down (in G) along with Tuba Skinny from their 2010 CD.
Today I saw the video made of them playing this tune at The Louisiana Music Factory in 2015 (Click here to watch it). I thought I would again play along. But this time they were playing it in F.
In the case of Lou's example, I think there is a simple explanation. In 2010, Erika was comfortable singing her song in G (one tone higher than in 2015). But her voice matured over the following five years. By 2015 her perfect comfort zone for a tune such as Six Feet Down had become the key of F. In that key, the lowest note used in her vocal is C and the highest is A, so (unlike some of Erika's other songs, such as Crazy Blues, where she sings high Ebs) it does not require a very great range - just four and a half tones. I am sure she could still sing it in G easily enough; but in F it sounds absolutely right for her 2015 'mature' voice.
Tuba Skinny are well-known for the freedom and boldness with which they roam around the keys and often change key (sometimes more than once) within a tune. On some occasions, the band plays a tune in one key and Erika - when taking a vocal chorus - sings it in another. For example, in How Do They Do It That Way?, you find the band playing choruses in Eb and Erika singing choruses in Bb. The transitions are so skilfully managed that you hardly notice. The same sort of thing happens in Delta Bound, with Erika singing in D minor and the band choruses in G minor.
Traditional jazz musicians come to learn that there is no such thing as a correct key for any tune. You can play in a band that performs Muskrat Ramble in Ab, for example, and then deputise in another band, only to find it plays Muskrat Ramble in Bb. A tune such as Ain't She Sweet may turn up in Bb or Eb. You will hear Breeze in either Eb or F. And so on. Whenever there is a singer, the whole band may have to adapt to an unusual choice of key. For example, after years of playing I Can't Give You Anything But Love in F, you one day find yourself in a band with a lady singer who requires the tune to be played in Bb.