Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of the first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing a new design according to a first embodiment; FIG. 2 is a second image thereof; FIG. 3 is a third image thereof; FIG. 4 is a fourth image thereof; FIG. 5 is a front view of the first image in a sequence of a display screen or portion thereof with transitional graphical user interface showing the new design according to a second embodiment; FIG. 6 is a second image thereof; FIG. 7 is a third image thereof; and, FIG. 8 is a fourth image thereof. The broken line perimeter in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the figures illustrate portions of the graphical user interface that form no part of the claimed design. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 4 and 5 - 8 respectively. The process or period in which one image transitions to another image forms no part of the claimed design.