MyBB Community Forums

Go to ACP -> Templates & Style -> Templates -> *Chose Your Theme Template* -> Post Bit Templates
Now, find "Postibt_Classic" and open it. Copy all content from there and close it.
Now Find "Postbit" and paste content you are copied (Postbit_Classic content)

Done!

Important: It does not matter what is checked in your panel
("Display posts using the horizontal post layout" or "Display posts using the classic layout") this will work anyway!

Or an SQL query

UPDATE `mybb_users` SET `classic_postbit` = '1' WHERE 1=1;

Yes, users could change it away, but most wouldn't. And those who do really don't want the forced postbit change.