Multiple different panel size connection

In the "olden days", yes, folks simply added more batteries to their system to "increase power". Batteries were cheap, and solar panels expensive.

The real answer is batteries "store energy" and solar panels "generate energy". Adding batteries will help you over 2-3 days of "no sun"--But they will not help if your daily loads (on average) exceed the solar harvest.

So, now we are at the "add panels" option. More panels, more energy harvest, and batteries are kept at higher states of charge (discharging below 50%--Roughly--Reduces cycle life. Letting batteries sit for days/weeks/months at below 75% sulfates them and pretty quickly they permanently lose capacity).

Today, solar panels are (relatively) cheap (15+ years ago, $10 per Watt, today around $0.50 to $1.00 per Watt)--And batteries are relatively expensive. So more panels is both technically and cost effective to give you more energy.

Yes, you can keep the panels all in series for the present for your PWM controller (12 volt battery bank = Vmp~18 volt array; if 24 volt battery bank then Vmp-array~35-40 volts for a PWM controller). (I got lost regarding if you have a 12 volt or 24 volt battery bank)

Yes, you can (usually) wire up a new MPPT controller to a PWM wired array. You probably will harvest a bit less energy--And you don't get the advantages of a "high voltage" solar array (using higher voltage array, use smaller diameter copper wiring and/or the ability to put the array farther from your home in a more sunny location). You will not hurt anything.

With off grid solar--Knowing your daily loads is critical towards designing a cost effective solar power system. And with off grid solar, you will always have a larger array to carry you through winter / a few days of stormy weather--And have a buffer when you need more power (guests, etc.).

More or less, your array/predicted harvest should be ~2x larger than your planned daily energy needs. Of course, part of the equation is how much generator runtime you want (or don't)--Larger array, less genset runtime. It is just the nature of the beast.

There are some things that solar alone cannot resolve--Typically far north (dark winters) and shading you have no control over (tree cover, buildings, even deep valleys).

-Bill