Replace 'on' and back with one click

Add to .vimrc:

function! s:ToggleQuote()
    let q = searchpos("'", 'n', line('.'))
    let qb = searchpos("'", 'bn', line('.'))
    let dq = searchpos('"', 'n', line('.'))
    let dqb = searchpos('"', 'bn', line('.'))
    if q[0] > 0 && qb[0] > 0 && (dq[0] == 0 || dq[0] > q[0])
        execute "normal mzcs'\"`z"
    elseif dq[0] > 0 && dqb[0] > 0
        execute "normal mzcs\"'`z"
    endif
endfunction
nnoremap '  :call ToggleQuote()

If you move the cursor inside the quotation marks and press', then the quotation marks will change to double / single.

ps. the script uses vim-surround .