Can a monitoring company take over an alarm someone else installed?

Usually yes. A takeover means an approved company inspects an existing alarm, confirms it meets the required standard or specifies what needs changing, and then maintains and monitors it. It is normally much cheaper than replacing a working system, and it is the standard route when you move into a property with an alarm already fitted.

The inspection is the crux. The company has to be willing to put its own certification behind a system it did not install, so it will check the panel's age and type, detector coverage, whether documentation exists, and whether confirmed activation is achievable if you want police response.

Outcomes are usually one of three: taken over as-is; taken over after specified remedial work, which might be as small as repositioning a detector or as large as a new panel; or declined, typically because the panel is obsolete or unsupported.

If you have just moved in, be aware you may have no history for the system: no documentation, no engineer code, no knowledge of what has been changed. Say so up front, because it affects the survey.

A takeover does not inherit the previous occupant's URN automatically, and a poor false-alarm history at the address can still be relevant.