I'm installing new propane and CO detectors and am reminded that the recommendation is every 5 years. The date stamp on each of mine is 1999 - they have never been changed out and the EOL(end of life) /replace lights did not come on. I put the new date on the face of the detectors with a Sharpie.
Remember - propane mount is low, CO is high. I am not using the combo propane/CO detectors for this reason.
Where did you source your replacements? Are they the exact same physical size, or modification of mounting holes required? Wiring exact match, or mods needed?
Not hassling you, but anytime you do a little project like this, a few extra "details" (and photos) can significantly improve the value of the post.
Don't worry if you are covering what you might believe is "common knowledge". Our newer members often don't think (or know) to search the Forum for older posts on the same topic. Plus, the sources and prices for replacement parts continually change, making a "fresh" report more timely.
Every "What I Did" post adds to the cumulative knowledge base of the collective! ^.^d
I bought the same model propane detector, which is still current after 18 years. I was able to find it at Camping World, but their new CO detectors were too narrow to fit the original opening in the bathroom. I found this one on Amazon: Amazon.com: MTI Industries 65-542 12-V Flush Mount Hard Wired Carbon... (https://www.amazon.com/gp/product/B000B9181A/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1)