d
Let d- distance between any vertex and the center.

The potential at center before and after the charges are interchanged =

\(\frac{1}{4 \pi \epsilon} \frac{q}{d}+\frac{1}{4 \pi \epsilon} \frac{q}{d}+\frac{1}{4 \pi \epsilon} \frac{-q}{d}+\frac{1}{4 \pi \epsilon} \frac{-q}{d}\)

Field at center after interchanging the charges \(=4 \frac{1}{4 \pi \epsilon} \frac{ q }{ d ^2} \cos \left(\frac{\pi}{4}\right)\) from \(C\) to \(A\)

The direction of field has changed.