Display Screen or Portion Thereof with a Transitional Graphical User Interface

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

FIG. 2 . is a second image in the sequence of the display screen or portion thereof with a transitional graphical user interface;

FIG. 3 . is a third image in the sequence of the display screen or portion thereof with a transitional graphical user interface; and,

FIG. 4 . is a fourth image in the sequence of the display screen or portion thereof with a transitional graphical user interface.

The outermost broken lines illustrate a display screen or portion thereof and form no part of the claimed design. The remaining broken lines illustrate the graphical user interface and form no part of the claimed design. The appearance of transitional image sequentially transitions between the images shown in FIGS. 1 - 4 . The process or period in which one image transitions to another image forms no part of the claimed design.