question

The system will never know the state of charge of either of the batteries. Since the system is only expecting one battery it does not have the ability to track how much energy has gone into or out of either one or the other battery as you switch between the two.

There are a few ways to do this, but which generally not officially "supported".

In terms of configuration of the charger, if the new battery bank is Lithium as well, you should be able to have a charging regime that is compatible with both old and new batteries, so switching between the two without changing config.

As far as having two separate banks and switching between the two, you could fit a "1+2" isolation switch. This would let you select between Bank 1, Bank 2, have both 1 & 2 connected at the same time, or disconnect both.

Your BMV would get 'confused' when you are switching between the two, but the charger would not care and its charge would be whatever the battery bank connected needed.

I have two Battery Banks in my setup (3 batteries in Bank 1 and currently 1 in Bank 2), and to deal with the BMV question, I actually have dedicated BMV-700s on each bank so I can see precisely what each bank is up to. Because I use the banks connected together generally, I also have a BMV-712 that monitors the entire bank for a overall picture.

HTH.