To change two vertically split windows to horizontally…

To change two vertically split windows to horizontally split

Ctrl-W t Ctrl-W K
Horizontally to vertically:

Ctrl-W t Ctrl-W H
Explanations:

Ctrl-W t makes the first (topleft) window current
Ctrl-W K moves the current window to full-width at the very top
Ctrl-W H moves the current window to full-height at far left

http://stackoverflow.com/questions/1269603/to-switch-from-vertical-split-to-horizontal-split-fast-in-vim