Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a front view of the first image in a sequence for a display screen or portion thereof with transitional graphical user interface showing our new design; FIG. 2 is the second image thereof; FIG. 3 is the third image thereof; FIG. 4 is the fourth image thereof; FIG. 5 is another front view of FIG. 1 , showing the claimed subject matter separately for clarity of illustration; FIG. 6 is another front view of FIG. 2 , showing the claimed subject matter separately for clarity of illustration; FIG. 7 is another front view of FIG. 3 , showing the claimed subject matter separately for clarity of illustration; and, FIG. 8 is another front view of FIG. 4 , showing the claimed subject matter separately for clarity of illustration. The outer perimeter shown as broken lines in FIGS. 1 - 4 illustrates a display screen or portion thereof and forms no part of the claimed design. The broken lines of a square enclosing a gray area in FIG. 1 form part of the claimed design. The remaining broken lines in FIGS. 1 - 4 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 . The process or period in which one image transitions to another image forms no part of the claimed design. The gray shading in the figures illustrates a contrast of appearance.