Transpose looks a bit akward in terms of something like the mandolintab abc converter. I've not found a utility that would rewrite the file in a different key - which is sometimes wanted.

To use the Kesh Jig example, output could be transposed by wrapping the music in a \transpose:

  \header {
        encodingdate = "2018-06-19"
        title = "The Kesh Jig"
        subtitle = " "}
        
        
        voicedefault = \transpose g a {
         \relative d' {
         \key g \major \time 6/8
          \repeat volta 2 {
         \partial8
              d8 | g4.^"~"^"G" g8 a b | a ^"D" b a a b d | e ^"C" d d g^"G/B" d d | e^"Am" d b d^"D7/F#" b a \break
              g4.^"~"^"G" g8 a b | a^"D" b a a b d | e^"Am" d d g^"G" d b | a^"D/F#" g fis g4^"G" } \break
         \repeat volta 2 {
         \partial8
              a8 | b4.^"~"^"Em" d8^"G" b d | e^"C" g e d^"G" b a | b4.^"~"^"Em" d8^"G" b g | a^"D7" b a a g a | \break
              b^"Em" a b d^"G" b d | e^"C" g e d^"G" b d | g^"C" fis g a^"D7" g a | b^"G" g fis g4 }                 
         }
        }
  \score{
      <<
  
          \context Staff="default"
          {
              \voicedefault 
          }
  
      >>
          \layout {
          }
          \midi {}
  }
  

I think this is moving the notes up to A correctly but the chords don't change. Maybe there is another means of chords to find?