Hello
I would argue that the views are, over the whole strech, about equally as good on both sides. The lakes you will be passing (Lake Lugano, Lake Lucerne, Lake Zug and Lake Zürich) are even mostly on the left side when facing forwards and the mountains are on both sides.
I do not think that it is easily possible to change the allocated seats, but there is no obligation to reserve seats as soon as you enter Switzerland (this means after the stop in Chiasso). From then on, you can will probably find a free seat nobody has reserved (check reservation displays!) and you can just sit there.