Answer
The potential difference across $R_1$ is the same as it was previously.
Work Step by Step
Let $\Delta V$ be the potential difference across the battery.
Initially, the potential difference across $R_1$ is $\Delta V$
After $R_2$ is added in parallel, the potential difference across each resistor is $\Delta V$
Therefore, the potential difference across $R_1$ is the same as it was previously.
