Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is the first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 2 is the second image thereof; and, FIG. 3 is the third image thereof. The dashed broken line perimeter in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 3 . This process or period forms no part of the claimed design.