Display Screen or Portion Thereof with a Transitional Graphical User Interface

FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface showing a first image;

FIG. 2 is a front view showing a second image; and,

FIG. 3 is a front view showing a third image.

The appearance of the transition is sequential from FIG. 1 to FIG. 3 . The process or period in which one image transitions to another forms no part of the claimed design.

The dot-dash broken lines illustrate the display screen and form no part of the claimed design.

The other broken lines illustrate portions of the graphical user interface and form no part of the claimed design.