Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a first image in a sequence for a display screen or portion thereof with transitional graphical user interface, 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 image sequentially transitions between the images shown in FIGS. 1 to 5 . The process or period in which one image transitions to another forms no part of the claimed design. The outer perimeter broken lines in the figures depict a display screen or portion thereof and form no part of the claimed design. The remainder of the broken lines in the figures depicts portions of the graphical user interface that form no part of the claimed design.