I think you can do the job.

Before you replace the system make sure you have A/C power to each jack, a bad splice will make affected units think half the power has been removed and a new system will operate the same way.

CO and CO/Smoke combos have an end of life feature but I don't think they were available six years ago.

All units in a system must be compatible with each other, usually by manufacturer.

Thank you for the responses. I bought all new units just alike. Didn't get the CO combo, with no natural gas in home don't think they are necessary....may be wrong on that.
At any rate, I called our local fire department to ask about the best units to get and if they could recommend anyone to install for me. To my surprise they said they would come and install and they did yesterday afternoon! Two guys, one a volunteer and one on county payroll. Took them about 15 minutes. I paid close attention and bus driver, you are right, I could do it, and will in my house.
Just to pass it along, check with your local fire department, they too may have a program to help you with these things. In fact he offered me three of the battery operated smoke detectors as part of their program.
And again, tax dollars at work for ME.
Thanks again for your responses.
Murphy

That's a neat story Murphy. Here in Dallas, smoke alarms & wiring is a fire marshall thing. May be wrong, but I don't think NEC code deals with them.