A scenario switch is one input cell that picks which column of assumptions the model uses. Change 2 to 3 and every number downstream recalculates for the high case.
=CHOOSE(index, value1, value2, ...) returns the value at position index: CHOOSE($B$1, B4, C4, D4) gives B4 when B1 is 1, C4 when it's 2, D4 when it's 3.
=INDEX(B4:D4, $B$1) does the same from a range, and grows more gracefully when you add a fourth case. With named scenarios in a drop-down, INDEX(B4:D4, MATCH(B1, B3:D3, 0)) switches by name instead of number.
Lock the switch cell with $ so the formulas fill down the column of assumptions.