Change over new 2017 2507s battery to two 6v
Upgrading batteries will not themselves require heavier gauge wire. You are getting more storage capacity with them, but don't need heavier wire to take advantage of the extra storage capacity.
There are other compelling reasons to get heavier gauge wire like:
1. You will want big enough wire to handle the amps. Unless you are adding an inverter which can draw a lot from the batteries (which is another subject), the only concern is charging amps from the converter. Chances are you are fine unless you upgraded the converter with a greater amp output.
2. Minimizing voltage drop due to distance between batteries and converter. Voltage drop will simply increase charging time.
Most folks that are concerned with above are boondockers who recharge their batteries with a generator and want to do it in as short a time as possible.