Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence, showing our new design in a first embodiment; FIG. 2 is a front view showing a second image in a sequence thereof; FIG. 3 is a front view showing a third image in a sequence thereof; FIG. 4 is a front view of a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence, showing our new design in a second embodiment; FIG. 5 is a front view showing a second image in a sequence thereof; FIG. 6 is a front view showing a third image in a sequence thereof; FIG. 7 is a front view of a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence, showing our new design in a third embodiment; FIG. 8 is a front view showing a second image in a sequence thereof; FIG. 9 is a front view showing a third image in a sequence thereof; FIG. 10 is a front view of a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence, showing our new design in a fourth embodiment; FIG. 11 is a front view showing a second image in a sequence thereof; FIG. 12 is a front view showing a third image in a sequence thereof; FIG. 13 is a front view of a display screen or portion thereof with transitional graphical user interface showing a first image in a sequence, showing our new design in a fifth embodiment; FIG. 14 is a front view showing a second image in a sequence thereof; and, FIG. 15 is a front view showing a third image in a sequence thereof. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 3 , FIGS. 4 - 6 , FIGS. 7 - 9 , FIGS. 10 - 12 , and FIGS. 13 - 15 , respectively. The process or period in which one image transitions to another forms no part of the claimed design. The outer perimeters illustrated by broken lines in FIGS. 1 - 15 depict a display screen or portion thereof and form no part of the claimed design. The remaining broken lines depict portions of the graphical user interface that form no part of the claimed design.