Display Screen or Portion Thereof with Transitional Graphical User Interface

FIG. 1 is 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; and,

FIG. 3 is the third image thereof.

The outer perimeters illustrated by broken lines in the drawings depict 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.

The appearance of the graphical user interface sequentially transitions between the images shown in FIGS. 1 - 3 respectively. The process or period in which one image transitions to another image forms no part of the claimed design.