Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is the first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 2 is the second image thereof; FIG. 3 is the third image thereof; FIG. 4 is the fourth image thereof; and, FIG. 5 is the fifth image thereof. The outermost broken lines in the drawings represent a display screen or portion thereof, and form no part of the claimed design. The remaining broken lines in the drawings depict portions of the transitional graphical user interface that form no part of the claimed design. The appearance of the transitional graphical user interface sequentially transitions between the images shown in FIGS. 1 - 5 . The process or period in which one image transitions to another image forms no part of the claimed design.