The reason for expiration dates on the carseats is that the materials deteriorate with everyday wear and tear on the seat. Even though you know the complete history of that seat, you do not know the strength testing or how plastic deteriorates internally. The true test of how a carseat works can only be evidenced by what it was designed to do, and that is to increase a child's safety in a crash. Just because others have used a carseat past the expiration date and was fine (was it ever in a crash, or just used in the car?), does not mean that your old carseat will work the way it was intended if you were to be in a crash. If you want to take that risk, that is your choice, I personally wouldn't.
Since they are only in an infant carrier type seat for a short period of time, I would suggest you use that one until the expiration date or your new baby outgrows it, whichever comes first, and then go to a convertible carseat. Or you could just buy a convertible carseat. I personally hated the infant carriers and never used one with my 2nd son after my 1st son outgrew his at 4 months.