It is possible to change an FMS if it crashes and hook up the call to another FMS, However the process is slightly elaborate and needs some experience with FMG control APIs
When a call drops due to FMS application unload; FMG detects that. If <maxHoldTime> in fmsmg.xml contains a non-zero duration in millisecond, the SIP call-leg will be kept preserved in hold state for the same duration. During this hold state, a special purpose FMG control application monitoring these events can fork a new callLeg to new RTMP destination and bridge the callLeg on hold with new destination.
If FMG crashes, call cannot be retrieved.
Frankly speaking, it is rare to find FMS OR FMG crashing in most applications; so most application are found to work well without such measures.