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