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; FIG. 2 is a second image thereof; FIG. 3 is a third image thereof; FIG. 4 is a fourth image thereof; and, FIG. 5 is a fifth image thereof. The appearance of the transitional graphical user interface transitions between the images shown in FIGS. 1 through 5 . The process or period in which one image transitions to another forms no part of the claimed design. The outermost dash-dot broken lines in the drawings illustrate the display screen or portion thereof and form no part of the claimed design. The dotted broken lines in the drawings illustrate portions of the graphical user interface that form no part of the claimed design.