If a circle can be inscribed in a parallelogram how will the parallelogram change?

If a circle can be inscribed in a parallelogram it will be a rectangle.

Let ABCD be a parallelogram inscribed in a circle.
Opposite angles of a parallelogram are equal

So ∠𝐵A𝐷 = ∠𝐵𝐶𝐷

Pair of opposite angles in a cyclic quadrilateral are supplementary ∠𝐵A𝐷 = ∠𝐵𝐶𝐷

$\begin{array}{r}=\frac{{180}^{\circ }}{2}\\ ={90}^{\circ }\end{array}$

Similarly the other two angles are also 90° and opposite pair of sides are equal
Therefore, 𝐴𝐵𝐶𝐷 is a rectangle.