will soldering in a new carbon monoxide sensor extend its life?

I am a big proponent of extending electronics usually - however a life safety device that protects you and your family should not be altered but should be replaced per manufactures specs - obviously you are concerned about the expired alarm - just changing the sensor may not help -I understand your decorum issue -

"Catalin,

They are software locked so they will all “expire” even if you solder a new carbon monoxide sensor. I never ended up doing because the software would think it expired regardless of whether the co sensor was working or not."

Don’t paint a smoke detector you could end up damaging the sensors. Gen 1 smoke detectors have a life of 7 years while the gen 2 have a 10 year life. Swapping the CO sensor would technically allow you to extend it’s life (Provided you can find the part and can calibrate it properly) but the expiry data is programmed into the unit itself so that won’t work. Swapping the guts is one option, provided that the gen2 will fit in a gen1 case, but you risk damaging the electronics etc. Your best bet would be 3D printing a case that will fit around the new smoke detector without interrupting the air flow, light, or speaker etc.