So yeah you can rewire Ademco. I’ve got no experience with those panels however, but all alarm panels basically just use a motion sensor or contact in either normally open or normally closed (look at circuits, for alarms it’s usually NC), plus a specified resistor for supervision (ie: don’t cut the wire and twist it together to trick it). The panel will be monitored either over a landline or via Ethernet if you buy an upgrade kit. Pretty simple machines, software is sorta hidden meant for only dealers, and a pain, but it’s actually pretty simple once you get it. You just sign up with a monitoring service and give them a zone list and they give you an account number.

If you replace it, or want to expand I’d probably do a NAPCO system since they are basic as fuck and they have a super simple wireless system that’s like stick contacts on doors or windows and your running in 30 minutes (mostly).