You'd need a lot of switches, and if you accidentally leave one in the wrong position while switching another one, KAPOW.
And first you have to switch them all to their "disconnected" positions, before you can switch any of them to the other mode from what they had been before.
Same thing with relays, except that at least that would be automatic. However, relays can disconnect or bounce under vibration, and if this happens under load, it could arc and destroy the relay if it isn't rated to handle that. (normally you wouldn't need to rate such relays very high, because you never need to switch them under load)
it's much much easier to use a plug to switch configurations. You'd need one with as many pins as you ahve packs and cell interconnects, if you want to balance this way.
You'd use larger pins/plugs for the main pack terminals that get seriesed, and smaller for the balance wires as thse don't normally see any significant current.
The seriesing one is a small plug, as it only has to jumper between the packs to put them in series. If you like you can also parallel balance connectors of the appropriate packs if it's higher than a 1p pack.
The paralleling one would be a bit bigger becuase it would also have the wires on it that go out to the charger.
This has been done in various ways by others, but I can't think of what search terms would find those threads easily.
