TransposeChordsUp.lua

local selectedMediaItems = reaper.CountSelectedMediaItems(0)

local notes = { 'A', 'A#', 'B', 'C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'G#' }

function transpose(amount, note)
  local index = 0
  for i, v in ipairs(notes) do
    if v == note then
      index = i
    end
  end
  
  if index ~= 0 then
    local newIndex = (index + amount - 1) % #notes
    return notes[newIndex + 1]
  end
end


for i=0,selectedMediaItems-1 do
  local mediaItem = reaper.GetSelectedMediaItem(0, i)
  local note = reaper.ULT_GetMediaItemNote(mediaItem)
  note = string.gsub(note, "([A-G]#?)", function(n)
    return transpose(1, n)
  end)
  reaper.ULT_SetMediaItemNote(mediaItem, note)
end

reaper.UpdateArrange()