Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of the first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 2 is the second image thereof; FIG. 3 is the third image thereof; FIG. 4 is the fourth image thereof; FIG. 5 is the fifth image thereof; FIG. 6 is the sixth image thereof; FIG. 7 is the seventh image thereof; FIG. 8 is the eighth image thereof; and, FIG. 9 is the ninth image thereof. The outer perimeter shown in broken lines in FIGS. 1 - 9 illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in FIGS. 1 - 9 illustrate portions of the graphical user interface that form no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 9 . The process or period in which one image transitions to another image forms no part of the claimed design.