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;

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof;

FIG. 4 is a fourth image thereof;

FIG. 5 is a fifth image thereof; and,

FIG. 6 is a sixth image thereof.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-6 . The process or period in which one image transitions to another forms no part of the claimed design. The broken lines around the figures show a display screen or portion thereof and form no part of the claimed design. The broken lines within the figures show portions of the transitional graphical user interface that form no part of the claimed design.