change sx2 mode to traktor.

It should do it automatically.
The first DDJ-SX you had to do it manually.
Taken from the user manual:
The unit automatically changes operation as follows depending on whether Serato DJ is running.
! When Serato DJ is running, controller operation depends on the Serato DJ
functions.
! When Serato DJ is not running, the unit operates as a general MIDI controller. Even though Serato DJ is running, you can forcefully operate the unit as a general MIDI controller.