Inspired by Steve answer, I wrote simple function that toggles between vertical and horizontal splits for all windows in current tab. You can bind it to mapping like in the last line below.
function! ToggleWindowHorizontalVerticalSplit()
if !exists('t:splitType')
let t:splitType = 'vertical'
endif
if t:splitType == 'vertical' " is vertical switch to horizontal
windo wincmd K
let t:splitType = 'horizontal'
else " is horizontal switch to vertical
windo wincmd H
let t:splitType = 'vertical'
endif
endfunction
nnoremap <silent> <leader>wt :call ToggleWindowHorizontalVerticalSplit()<cr>