I am new to MyBB, switching from SMF.
I want to know if it is possible to downgrade an existing MyBB 1.4.x forum to a MyBB 1.2 forum, and if so how.
It's slightly impossible - it's like asking someone using Vista to downgrade to Windows 98...
The only way is to restore a backup of a 1.2 board. But from what you're asking, I doubt you have one of those...
There are numerous benefits to having 1.4 over 1.2 - not to mention there won't be anymore support for 1.2 soon.
It's for a merge... Okay.
Yeah the only way to 'downgrade' is to revert to the last backup from that version. You can merge to 1.2.x anyway.