Just curious why you would want to do that?
In a parallel setup the negative terminals are tied together and the positive terminals are tied together. You get the same voltage as a single battery but increased capacity (mah) and current handling ability.
To make it a series mod you'd need to connect the positive of one battery to the negative of the other. The remaining terminals would be tied to the existing negative and positive wiring, positive to positive wire, negative to negative wire. You would get double the voltage with a capacity and current handling ability similar to a single battery. There are some issues running stacked batteries, but I'm no expert on them.
Probably would depend on the mod as far as how easy it is to convert.