New airframe - help with mixer modification

Hi All,
I need help with mixer modification for new airframe type. In fact, the drone is already flying. The problem is that after vertical take off I need transition to forward flight.
Currently the drone is configured as octocopter. Strange thing, but propellers on every arm of the drone blows in opposite direction. Now what I need is to change direction of all bottom propellers. I guess it is difficult to imagine but maybe this site will clarify how it looks like:
I shortly explain. On every arm there is pipe (there are 4 arms/pipes). At the beginning of the pipe and at the end there is propeller. Every pipe has hole at the bottom. To fly like a drone all propellers blow to the inside of the pipe so it creates force upward. And this is working. You can see it on the movie. This what I need now is transition to forward flight like a plane. To achive this I need to change direction of the back propellers on every pipe. Front propellers should still work in the same direction.
Can you give me any advice how to follow with mixer modification? Is it enough to modify configuration of the mixer or changes in the source code will be necessary?
Thank you in advance.

What kind of motor does it use? Normally the brushless 3 phase motors that drones use can not be reversed.

Thank you for your answer. We use this motors:
I am pretty sure there should be some way to change the direction of the motor.

Well you will need to discover how to do this as normally the ESC in it’s default state only goes one direction. Sure it’s possible, just need to figure out how.
The code does not know how to reverse a motor so there will need to be changes somewhere.
Also the rate controller which is the lowest level PID control does not understand reverse either.
Look thru this: