@achiral; great idea, that is what I am currently working on. Actually opened up the CO detector and wires were actually cut (someone must have gotten tired of an alarm). The front panel also said "replace by 2011." The Propane Gas Leak Detector also was mfg in September 2007. I'm obviously replacing both.
It looks like it is prewired. Found the wires behind the front access panel.
I finally ordered and installed a CO detector and propane detector based on the 'exact fit' charts in that link.
Unfortunately, exact fit doesn't appear to mean the same thing for FirstAlert as it does for me. The propane detector was approximately 1/4" less wide than the previous version. I was able to make this still work with the original cutout from 4WC as there was adequate luan material to screw into and the detector mounting flange was wide enough to hide the old screw holes.
The new CO detector, on the other hand, had an additional 3/8" depth compared to the old part as well as slightly larger vertical dimensions. Since there is minimal clearance between the rear luan paneling and rear frame, the new detector did not fit flush with the wall. I ended up slightly increasing the size of the cut-out in the luan (~1/2" vertical) and making an exact-fit 3/8" thick frame of plywood to fit the detector cut-out and flange. It looks extremely clean, but did add a couple of hours of work to the replacement of the old detectors.