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; and, FIG. 2 is a front view showing a second image thereof. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 2 . The process or period in which one image transitions to another image forms no part of the claimed design. The outermost broken lines illustrate a display screen or portion thereof and form no part of the claimed design. The remaining broken lines illustrate portions of a graphical user interface and form no part of the claimed design.