Display screen or portion thereof with transitional graphical user interface

FIG. 1 is a front view of the first image in a sequence of a display screen or portion thereof with transitional graphical user interface, showing our new design;

FIG. 2 is a front view of the second image thereof;

FIG. 3 is a front view of the third image thereof;

FIG. 4 is a front view of the fourth image thereof;

FIG. 5 is a front view of the fifth image thereof; and,

FIG. 6 is a front view of the sixth image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1 to 6 . The process or period in which one image transitions to another forms no part of the claimed design.

The outer perimeter broken lines in the figures depict a display screen or portion thereof and form no part of the claimed design. The remainder of the broken lines in the figures depicts portions of the graphical user interface that form no part of the claimed design. The contrast shown in grayscale in the claimed portion of FIGS. 1 - 6 represents a contrast of appearance.