Upmix 5.1 to 7.1

[noob00225]

Hello,

I would like to take a 5.1 audio source and upmixit to 7.1.

LAV audio with 7.1 mixer the just moves the Back L/R channels to Side L/R.
Disabled some of the options on LAV audio font panel, the ones related to surround sound, did not change anything.
Also tried the options on the Mixing panel, still same.

FFdshow leaves the 2 extra channels empty.
I can create a custom matrix here, but what will happen if I enable the 2 extra channels for 7.1 on a 5.1 source?

Using a PC with MPC HC.

Thank you.