Just climbed up a ladder, removed three old ceiling mounted smoke alarms and their mounting plates, used the same screws in situ to attach new mounting plates, installed three new smoke alarms.
This was NOT my idea - Maryland law now requires everyone to have sealed units with 10-year batteries in place as of January 1, 2018.
But I'm pretty tickled (chuffed, if you're in a Commonwealth country ), because I wasn't sure I'd be able to get up there and stay up there long enough to ceiling mount any of them, let alone re-use the existing mounting screws, and never mind doing it three times, reaching up over my head with both hands from the top of a ladder. (I have ten foot cathedral ceilings.)
Most of my neighbors have been wall mounting theirs (it's legal; ceiling mount preferred) and putting a cover where the old ones were, and they're handier than I am.
I have one more to do, but it's hardwired, and evening is closing in, so I am going to have to actually read the instructions on that one's replacement and I'm leaving it for tomorrow when the light is better. (Edit in: God, I love this stuff! I never knew!)
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"I believe you find life such a problem because you think there are the good people and the bad people," said the man. "You're wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides."
-- Terry Pratchett, Guards! Guards!