Display Screen or Portion Thereof with a Transitional Graphical User Interface

FIG. 1 is a front perspective view of a display screen or portion thereof with a transitional graphical user interface, showing a first image in a sequence of our new design; FIG. 2 is a front view showing a first image in a sequence thereof; FIG. 3 is a front view showing a second image thereof; FIG. 4 is a front view showing a third image thereof; FIG. 5 is a front view showing a fourth image thereof; FIG. 6 is a front view showing a fifth image thereof; FIG. 7 is a front view showing a sixth image thereof; FIG. 8 is a front view showing a seventh image thereof; FIG. 9 is a front view showing an eighth image thereof; FIG. 10 is a front view showing a ninth image thereof; FIG. 11 is a front view showing a tenth image thereof; FIG. 12 is a front view showing an eleventh image thereof; FIG. 13 is a front view showing a twelfth image thereof; FIG. 14 is a front view showing a thirteenth image thereof; FIG. 15 is a front view showing a fourteenth image thereof; FIG. 16 is a front view showing a fifteenth image thereof; FIG. 17 is a front view showing a sixteenth image thereof; FIG. 18 is a front view showing a seventeenth image thereof; and, FIG. 19 is a front view showing an eighteenth image thereof. The inner broken line rectangle in the figures shows the display screen or portion thereof. Broken lines outside the display screen show the environment of a computing device. The remaining broken lines show portions of the graphical user interface. All broken lines form no part of the claimed design. The design sequentially transitions between the images shown in FIGS. 2 - 19 . The process or period in which one image transitions to another image forms no part of the claimed design.