Closing the switch and thus opening a second path of equal resistance for the current has the same effect as reducing the resistor R2 by a factor of 2.
The total resistance is therefore decreased by a factor of (1+0.5):2 = 0.75.
Assuming no change in the potential difference across the voltage source, the current will therefore increase by a factor of 1:0.75 = 1.33 and the voltage drop across R1 will increase accordingly.
The current through R2, however, will decrease because only half of the increased current will pass through (1.33/2 is less than 1).

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