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 according to a first embodiment;

FIG. 2 is the second image thereof;

FIG. 3 is the third image thereof;

FIG. 4 is the fourth image thereof;

FIG. 5 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 according to a second embodiment;

FIG. 6 is the second image thereof;

FIG. 7 is the third image thereof; and,

FIG. 8 is the fourth image thereof.

The outer perimeter shown in broken lines in the figures illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines in the figures, including the gray broken lines, illustrate portions of the graphical user interface that form no part of the claimed design.

The solid line corner elements shown within the gray solid line quadrangles in FIGS. 3 - 4 and 7 - 8 form part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 - 4 , and 5 - 8 , respectively. The process or period in which one image transitions to another image forms no part of the claimed design.