Impossible to change 20C from Switch mode to Interface mode?

[AlexFeren]

I wish to change my FWF-20C-ADSL from Switch mode to Interface mode, however, the system global's internal-switch-mode is missing:

FW20CA3X13000939 # show full-configuration system global | grep internal     set internal-switch-speed auto auto auto

According to "FortiOS™ Handbook - What's New in FortiOS 5.2.4" section "Merge FGT-20C-ADSL and FWF-20C-ADSL", mentions only two modes: 1. ADSL + LAN (LAN1,LAN2,LAN3,LAN4 as one switch) 2. ADSL + LAN (LAN1,LAN2,LAN3 as one switch) plus WAN interface

So, it's not possible to set Interface mode?

PS.

FW20CA3X13000939 # diagnose sys checkused system.interface.name lan FW20CA3X13000939 #