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; and, FIG. 4 is the fourth image thereof. The outermost perimeter broken line in each drawing figure depicts a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the figures, including the gray broken lines, 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 - 4 . The process or period in which one image transitions to another forms no part of the claimed design.