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 first embodiment of a display screen or portion thereof with transitional graphical user interface, showing our new design; FIG. 2 is a front view of the second image thereof; FIG. 3 is a front view of the first image in a sequence of a second embodiment of a display screen or portion thereof with transitional graphical user interface, showing our new design; and, FIG. 4 is a front view of the second image thereof. The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 2 and between the images shown in FIGS. 3 - 4 , respectively. The process or period in which one image transitions to another forms no part of the claimed design. The outer perimeter illustrated by a pair of dashed broken lines in FIGS. 1 - 4 depicts a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines depict portions of the graphical user interface that form no part of the claimed design.