When installing/upgrading a Solution, Symantec Installation Manager fails to configure throwing 'unknown user name and/or password' errors in the logs.
Symantec Installation Manager uses its own cached credentials instead of the current Application Identity. If the Notification Sever credentials change, then Symantec Installation Manager can no longer install new products or updates.
It appears that SIM keeps its own cached copy of the credentials based off of either initial installation. There is no UI in SIM where the credentials could be changed.