Vim mailing list says (re-formatted for better readability):
To change two vertically split
windows to horizonally splitCtrl-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
Note that the t is lowercase, and the K and H are uppercase.
Also, with only two windows, it seems like you can drop the
Ctrl-W t part because if you’re already in one of only two windows, what’s the point of making it current?
In VIM, take a look at the following to see different alternatives for what you might have done:
Ctrl-W s Ctrl-W o Ctrl-W v Ctrl-W o Ctrl-W s ...