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; and, FIG. 2 is the second image thereof. The outer perimeter shown as dashed broken lines in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The thick black and gray broken lines in the figures form part of the claimed design. The remaining broken lines in the figures, including the thin gray 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. The light gray and dark gray shading in the figures illustrates a contrast of appearance.