changing hard wired smoke alarm on house that once had an active alarm system which is now shut down
OK, clarify. Are you referring to alarm system detectors, or hard wired line-voltage (120V) smokes?

Either way, if you shut off the main, you should have no line voltage power anywhere. Line voltage smokes can, and typically do, have battery backup (9V battery), but this would NOT pose ANY safety hazard with regard to working on them.

If you are talking about an alarm system, then there is also mostly likely a system battery backup, but again, this should pose no safety hazard at all since these systems run on low voltage anyway.