I had the DX9 profile selected, and wanted to try Ultra/DX11 x5.
Everytime I changed the settings to DX11, it said a restart would be required.
Restarting, always set the game to DX9 mode because that was my profile.

With a bit of testing, DX11 profile starts as DX11 no matter what you set it to prior to the restart.
Similarly, using the DX9 profile starts the character set with DX9 set when you launch.

If you use Default Profile, rather than a DX9/DX11 preset, the in-game setting can be changed and when you do a restart of the game it even remembers the changes.