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 our new design; and, FIG. 2 is a second image thereof. The outer perimeter illustrated by dashed broken lines in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The parallel, heavy line weight broken lines in FIG. 1 adjacent the stipple shading forms part of the claimed design. The remaining dashed 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 - 2 . The process or period in which one image transitions to another image forms no part of the claimed design.