Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of a first image of a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 2 is a second image thereof; and, FIG. 3 is a third image thereof. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 3 . The process or period in which one image transitions to another image forms no part of the claimed design. The outermost broken lines in FIGS. 1 , 2 , and 3 illustrate the display screen or portion thereof and form no part of the claimed design. The remaining broken lines illustrate portions of the graphical user interface that form no part of the claimed design.