Hi,
I change and save the option in admin cp, bit in the posts the horizontal layout is displayed.
Your problem is probably the setting in your usercp.
Usercp>Edit Options
Look under "thread view options" and check the "Display posts in classic mode." option.
The ACP setting is only for guests and new users, current users have to change it in their User CP. It's been like that since the horizontal postbit as added.