Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is a 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 a second image thereof.

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 outer perimeter shown in dashed broken lines in the figures illustrates a display screen or portion thereof and forms no 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.