ONLINE Changing ladder logic for a redundant PLC5

I need advise on a redundant PLC 5. I need to modify the ladder logic and add in some new data files for a redundant PLC5/40. The system is critical & cannot afford a shutdown.

I'm planning to change the program on the secondary (backup) PLC5/40 by putting the secondary PLC 5/40 into REM, after the modification is done, I'll switch it back to RUN. Then, I'll switch the PRIMARY PLC5/40 into REM & let the SECONDARY PLC5/40 take control, while I modify the PRIMARY PLC5/40 ladder logic.

After completed, I'll switch the PRIMARY to RUN also and switch the SECONDARY PLC5/40 to PROGRAM, so that the PRIMARY PLC5/40 will take control.

I've no experience with redundant PLC5 system, am I doing the right thing in order to avoid the machine to shutdown?

Hope someone can give me a word or two. Thanks

Is this using the BCM modules or the CHBM?

It's using BEM & BCM modules. Any differences? Thanks.

Anyone care to response? Thx.