There is no right answer for this problemgenerally. I'm not sure to be able to give You an understandable answer but I'll try...

Basically You have to consider all the conditions of Your special machine configuration and have to find out the easyest way to fix the problem.

1st step: disable the encoder in the CNC:

- set in the specified axis MD ENC_TYPE[1] = 0 and/or MD NUM_ENCS = 1

- execute a NCK-reset ==> 2nd encoder is not connected to the CNC anymore

2nd step: switchover the positioning loop to the 1st encoder:

- deactivate the PLC.interface signal for 2nd encoder by setting DB(specified axis).DBX1.6 = FALSE and activate the PLC.interface signal for 1st encoder by setting DB(specified axis).DBX1.5 = TRUE

therefore You have to check (and perhaps to modify) the PLC-ladder program (STEP7 needed). Some machine manufacturers already prepared the PLC-ladder program with a function for encoder deactivation/switchover. mostly by using MD PLC_USER_DATA_INT/orHEX or definition in an independend DataBlock in the PLC. ==> difficult to give You a detailed information from far away...

- also possible instead of the PLC-modification: swapping of MD ENC_NR[0] = 2 (orig: 1) and MD ENC_NR[1] = 1 (orig: 2), need to keep MD NUM_ENCS= 2.

3rd step: preparation for referencing the axis:

pending on the original machine configuration (CNC, attached encoders, designed gear boxes/belt drives) the referencing of the new modified axis becomes quiet tricky or is not possible (missing reference cam, crazy ratio in axis gear box) ==> no generaly solution available

4th step: consider the machine application:

the machine application refers internally to the value from 2nd encoder ==> it's almost impossible to cover these kind of impacts (fortunetaly it's rarely in the field...)

5th step: SAFETY:

pending on the application the SAFETY-parameters might be to modified and acknowledged as well. But in Your case (840Di) the SAFETY is not available as I remember.