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.