gistfile1.vim

"
" Works in visual line and visual block mode.
"
" Select one character more in visual block mode...
"
function! Transform(prefix, suffix)
    let [startcol,  endcol ] = [col("'<"),  col("'>") ]
    let [startline, endline] = [line("'<"), line("'>")]

    let startcol = '%'. (startcol ? startcol : 1) .'c'
    let endcol   = endcol ? ('%'. (endcol) .'c') : '%10c'

    execute startline.','.endline.'s/\v'.startcol.'(.*)'.endcol.'/'.a:prefix.'\1'.a:suffix.'/'
endfunction

vnoremap <leader>M :<c-u>call Transform('<% ', ' %>')<cr>