Running an old battery in parallel with a new one
Others will have their own opinion but here's what I think:

Provided that your existing AGM battery is capable of keeping a charge, there isn't any downside to leaving it in the system.

However, if your existing battery has significant self-discharge, you are best to remove it.  

One of the failure modes of Lead-Acid batteries is that one or more cells can develop internal short circuit paths that result in varying amounts of self-discharge current.  If your existing battery maintains its voltage above 12.5 Vdc for a week or more while sitting disconnected from anything else, it should be good.

However, if the battery voltage drops to a voltage significantly-lower than 12.5 Vdc, time to send it off to the recycle center.

The bottom line is that adding capacity to your system is a good thing.  If the existing battery is in good shape but simply has lost a portion of its capacity, you are better off having it in the system.