SOLAR - Switch from PWM to MPPT, series vs parallel matter?

My PWM controller is a Morningstar Tristar 45. It has a lot of features and I think works well. I too designed initially for expansion but found that after upgrading to 2x6V GC batteries that I didn't need any more solar to meet our needs. But some of these 40A MPPT controllers on Amazon have temperature compensation and are fully programmable as well for under $200. Thus the temptation...