Display screen or portion thereof with transitional graphical user interface

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

FIG. 2 is a second image thereof.

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

The longer outer-most broken lines showing an electronic device illustrates environmental subject matter. The regular broken lines showing a display screen and elements of the graphical user interface including text and symbols illustrate portions of the article. None of the aforementioned broken line subject matter forms part of the claimed design.

The heavy broken lines shown as rows of horizontal rectangles form no part of the claimed design. The narrow double lines, including a broken line segment at the left end of the lower line, do form part of the claimed design.