Just run the cart on 42v all the time and forget the switch.
You are right on the edge of the controller if it's 36v, most of them may handle an additional 6v but they may also fail.
A 36v charger won't charge a 42v pack, it will charge a 36v pack and you will need to use a 6v charger for the last battery.
You can either use your solenoid on 42v or shortwire it to 36v on the pack.
If you have an LED or analog state of charge meter, it will also have to be shortwired to 36v.
42v will get you about 15% more speed and torque.
One more thing if you decide to go to 42v, I have a 42v Lestronic ll charger for sale.
I've tried the extra 6 volt battery to jump to 42V but when I depress the pedal all I'm getting is a clicking from my solenoid. Do I need to go to a HD36V solenoid or jump to a 48V solenoid?
Can you start a new post so this one doesn't get lost.
I will give you the lowdown on 42v there.