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;

FIG. 3 is the image of FIG. 1 with additional unclaimed environmental subject matter shown in broken lines; and,

FIG. 4 is the image of FIG. 2 with additional unclaimed environmental subject matter shown in broken lines.

The broken lines showing of the display screen in the figures and the device in FIG. 3 and FIG. 4 are for the purpose of illustrating environmental structure and form no part of the claimed design.

The appearance of the transitional image sequentially transitions between the images shown in FIGS. 1-2 and between the images in FIGS. 3-4 . The process or period in which one image transitions to another forms no part of the claimed design.