@Tim > If you had a rekordbox 5 license key included for free with the hardware, you're able to use those same functions in rekordbox 6 for free as well. Nothing changes there.
CDJ-2000NXS2's never came with a license, nor did DJM-900NXS2's, so they are only eligible to use rekordbox 6 with a plan - same way they were with rekordbox 5. The reason those units never shipped with one is they're not intended as controllers, thus the expected use for rekordbox is to prepare and export your music to a USB drive, or link to them.
A DDJ-400, however, is useless without performance mode.