Siemens SIMATIC S7-400H Manual page 131

Preparations
To keep the time during which the fault-tolerant system must run in Solo mode to a
minimum you should perform the following steps before commencing the
hardware modification:
Make sure that the memory components of the CPUs are sufficient for the new
configuration and the new user program. If necessary, first expand the memory
components (see section 10.7).
Make sure that modules that are plugged in but not configured do not have any
effect on the process.
S7-400H Programmable Controller Fault-Tolerant Systems
A5E00068197-04
Modifications to the System while in Operation
10-5