I would replace them as suggested by the manufacturer. One possibility
to look at is whether your hard-wired detectors are interconnected.
When I was rehabbing a 3-story house, I put detectors on each floor,
hard-wired, and interconnected. One went off - they all went off.
(Yes, it was LOUD). I couldn't find battery backup detectors at the
time - early 90's - either they weren't available or I didn't search
enough. So I also installed battery-powered detectors on each floor
(not interconnected). Even for all those detectors, I spent less than
$200 - cheap protection for multiple lives and a $150K house.

But I would think that hard-wired, battery-backup, interconnected
detectors would be the best. Overly cautious perhaps, as I've never
set a house on fire, but you never know.

steve