Nortel Meridian 1 Manual page 799

During the upgrade procedures, the following service interruptions may
occur:
— For AC-powered systems, power to the entire column being upgraded
must be turned off for safety reasons. This causes loss of service to any
network, I/O, or service cards located in that column.
— Transient calls, CDR information, PRI service, and traffic report
information may be lost.
When the peripheral software download feature is activated, calls over
superloops are dropped during initialization.
Upgrade system installation Upgrading to Meridian 1 system options
Upgrades to Meridian 1 option 81 12-35