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 outermost even-dash broken lines illustrating a display screen or portion thereof form no part of the claimed design. The remainder of the even-dash broken lines illustrating portions of the transitional graphical user interface forms 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.