I believe the old battery will not hold it's peak voltage and will pull the new one down with it. So you will lose effective capacity on the new battery.
My main solar battery when brand new would hold 13.1V for 24 hours after charge. Now, about 300 50% cycles in it drops to 12.9V within a few hours.
You could test both your batteries, by boost charging them for an hour, then floating them for a few hours and finally leaving them overnight to see how they hold their voltage.